ASHINGTON, Sept. 15 ・President Bush told the American military today to get ready for a long war against terrorism, and vowed to "do what it takes to win."

In a brief appearance with his senior advisers at Camp David, where they met to plan the new offensive, Mr. Bush said point-blank: "We're at war. There's been an act of war declared upon America by terrorists, and we will respond accordingly."

"My message is for everybody who wears the uniform to get ready," Mr. Bush said.

Shortly afterward, in his weekly radio address, he warned that "those who make war on the United States have chosen their own destruction." He told Americans to steel themselves for "a conflict without battlefields or beachheads."

Victory, he said, "will not take place in a single battle, but in a series of decisive actions against terrorist organizations and those who harbor and support them."

While some of his remarks were scripted and others were delivered off the cuff, they demonstrated an escalation of harsh words that Mr. Bush used Tuesday night in an address to the nation after the terrorist attacks in New York and at the Pentagon. His language was backed up by a resolution passed overwhelmingly by Congress on Friday approving the use of force in response to the terrorist attacks. And the words and tone reflected the national mood.

Americans say overwhelmingly that the nation should take military action against those responsible for the terrorist attacks, the latest New York Times/CBS News Poll shows.

Mr. Bush identified Osama bin Laden, a Saudi-born millionaire harbored in Afghanistan by its radical fundamentalist government, as a "prime suspect" in the attacks that may have killed more than 5,000 people.

But Mr. Bush would not describe the administration's intelligence or its plans.

The administration also pressed ahead on the diplomatic front, campaigning through its envoys around the world to build a solid international coalition of friends and isolate foes in waging what Mr. Bush is bluntly calling war.

In an unusually strong diplomatic intervention, Secretary of State Colin L. Powell has asked Saudi Arabia, America's closest ally in the Persian Gulf, and the United Arab Emirates to sever diplomatic relations with the Taliban in Afghanistan, senior administration officials said today.

The Taliban, the extremist Sunni Muslim group that rules most of Afghan territory, has given refuge to Mr. bin Laden and has allowed him to maintain military training camps on Afghan soil.

The State Department directed its envoys in Saudi Arabia and the United Arab Emirates to deliver the request, the officials said. The Bush administration does not yet have ambassadors in place in either country.

As terrified Afghan citizens began leaving their country and neighboring Iran began to seal its borders against a wave of refugees, the Taliban leaders threatened a holy war against all those who helped in an American-led military campaign against their country.

But Pakistan today offered the United States the support it was seeking. Mr. Powell told reporters at Camp David that Pakistan agreed "to assist us in whatever might be required" in retaliating.

The United States has asked Pakistan to allow American access to Pakistani airspace, a vital consideration in any air strike on bordering Afghanistan; grant access to information from Pakistani intelligence on Mr. bin Laden; help track Mr. bin Laden's Al Qaeda organization and close off the organization's financial assets; and tighten the illegal flow of fuel and other supplies over the rugged mountainous border.

Pakistan's foreign minister, Abdul Sattar, today pledged cooperation but said that any military action would have to be supported by the United Nations.

"The government will discharge its responsibilities under international law," Mr. Sattar said in Islamabad, adding that he did not expect Pakistan to take part in military operations outside its borders.

Attorney General John Ashcroft, who was at Camp David with Mr. Bush, confidently said that the investigation was proceeding "with reasonable success" and that investigators were "beginning to understand the ways in which this terrible crime was committed."

After passing the resolution on use of force on Friday, Congress was out of session for the weekend. Members predicted that when they returned they would swiftly resolve the remaining budget issues before the fiscal year ends on Sept. 30. Many also predicted that Congress would put aside most of the domestic agenda until next year so as not to engage in partisan debate over issues like health care and campaign finance revisions.

In the skies of Washington, New York and other cities, F-15 and F-16 fighter jets continued to fly patrols they began on Tuesday after the attacks on the World Trade Center and the Pentagon. The fighter jets are supported by AWACS surveillance aircraft. Coast Guard cutters cruised ports and waterways on the Atlantic and Pacific coasts, where only essential cargo vessels were allowed to dock.

The cockpit voice recorder from the hijacked jet that left Newark and crashed in western Pennsylvania on Tuesday was brought to Washington overnight, to a specially equipped laboratory of the National Transportation Safety Board, where technicians were examining it this morning under the supervision of agents of the Federal Bureau of Investigation.

The Federal Aviation Administration said that Logan International Airport, where the two planes that hit the World Trade Center on Tuesday took off, reopened at 5 a.m. today. That left Ronald Reagan National Airport in Washington as the last major airport in the country still closed to all flights. Airplanes landing and taking off there fly along the Potomac River very close to Capitol Hill, the White House and the Pentagon, which was hit by the fourth hijacked jet. That one had left Dulles International Airport, in Virginia, which has reopened.

The aviation agency said that at 1:50 this afternoon, there were 4,950 planes in the skies nationwide. That is about 10 percent more than were flying when the hijackings occurred last week.

In the first weekend after the bombing, Washington was making its best attempt to move closer to normalcy, with the police retreating from heavy security and office buildings glinting with American flags draped informally, passionately from office windows. The Humvee roadblocks near the White House were gone, and there were no longer troops in combat fatigues controlling traffic at nearby intersections. A few early risers returned down Pennsylvania Avenue to get a good look at the president's home, which officials have said might have been an intended target in the coordinated attacks by hijacked jets.

"It's kind of solemn to see how few people have been in the streets," said Krista McFaren, a jogger who bounded by from DuPont Circle. "Everybody's been glued to the TV, feeling a kind of a sense of helplessness, spreading the sadness."

On the White House lawn, a television reporter talked into the camera, his words sounding lost and weary. A homeless man sipped morning coffee on a park bench facing the White House.

As construction crews prepared to remove heavy debris from damaged sections of the Pentagon today, Defense Department officials announced that they had awarded a $145 million contract to begin renovation on the rest of the 6.5-million- square-foot building. Those improvements, which are intended to protect against terrorist attacks, will include installation of blast-resistant windows, fire sprinklers and new steel columns. Similar renovations that were recently completed on the western side of the Pentagon, where hijackers crashed a jetliner into the building, helped save hundreds of lives by preventing damaged areas from collapsing and fire from spreading, Pentagon officials said.

In the seclusion of Camp David, in a remote part of Maryland, Mr. Bush woke up at 5:30 a.m., walked his dogs, went for a run, called the leaders of Spain and Mexico, attended his daily intelligence briefings, and gave his radio address. The meeting of the National Security Council began at 9:30 and ran until noon. The president and his advisers had lunch together, and planned to dine with their spouses this evening. No formal meetings were scheduled for the afternoon.

On Wall Street in New York, leading executives from exchanges and trading houses found success in testing whether the electrical and telecommunications systems were sufficiently repaired to allow the stock exchange to reopen on Monday. As Mr. Bush ordered flags to be flown at half-staff for another week, federal agencies continued to expand their contribution to rescue efforts in New York. The Centers for Disease Control and Prevention was sending 35 members of its Epidemic Intelligence Service to assist the New York City Health Department in the monitoring of public health matters.

But the biggest question continued to be the nature of the military campaign that the administration is putting together.

"Victory against terrorism will not take place in a single battle but in a series of decisive actions against terrorist organizations and those who harbor and support them," Mr. Bush said. "We are planning a broad and sustained campaign to secure our country and eradicate the evil of terrorism."

He called those responsible for the terrorist attacks "barbarians."

The United States has not specifically asked Pakistan to allow the deployment of American troops on Pakistani soil, but it is understood by the Pakistani leadership that it might have to do so, senior administration officials said. Such a move is likely to be opposed by Pakistan's military, Pakistani officials said.

"We have never had foreign forces operating from our soil," a senior Pakistani official said in a telephone interview from the region. "A foreign military presence could cause serious domestic problems for our leadership and and could create a serious backlash."

Pakistan is the only other country, other than Saudi Arabia and the United Arab Emirates, to recognize the Taliban as the legitimate leader of Afghanistan. The United States has not asked Pakistan to cut ties with the Taliban ・at least not yet ・recognizing that Pakistan has steadfastly maintained relations with the government in Kabul even when it helped the United States and Afghan rebels fight a covert war for years after the Soviet Union invaded Afghanistan in 1979.

The request could put both countries, but particularly Saudi Arabia, in a delicate position. Saudi Arabia's cooperation is crucial in mounting any military response in the region.

Saudi Arabia rescinded Mr. bin Laden's citizenship in 1994 because of concerns about his mounting appeal and his wealth. But for years it turned a blind eye to the financing of Mr. bin Laden by wealthy Saudi businessmen.