ASHINGTON, Sept. 25 ・President Bush came close to telling the Afghan people today to overthrow the Taliban government, encouraging them to rid Afghanistan of what he called "an incredibly repressive" administration.

Mr. Bush, answering questions at the White House, said the best way to bring those responsible for the Sept. 11 terrorist attacks to justice was "to ask for the cooperation of citizens within Afghanistan who may be tired of having the Taliban in place."

But an hour after Mr. Bush spoke, the White House press secretary, Ari Fleischer, interpreted the president's remarks as stopping short of a call for toppling a sovereign state. The planned military action in the region, Mr. Fleischer said, "is not designed to replace one regime with another regime."

The administration, which has been critical of "nation building" missions for the United States military, appeared to be trying to walk a fine line between increasing pressure on the Taliban, who have given refuge to Osama bin Laden, the accused terrorist, and finding itself enmeshed in an eventual attempt to rebuild the ruined state of Afghanistan under a new government.

Washington has accused Mr. bin Laden and his Al Qaeda network of terrorists of being responsible for the attacks on the World Trade Center and the Pentagon. Last week, the president said the Taliban had to "hand over the terrorists or they will share their fate."

Since the attacks, the administration has stepped up contacts with the Northern Alliance, a loose anti-Taliban coalition that controls 10 percent of Afghanistan. But any attempt to install the alliance in Kabul would be fiercely opposed by Pakistan, whose support is essential for United States military plans. [Page B5.]

The president seemed mindful of that. While calling on Afghans to cooperate against the Taliban, he said he was not interested in creating a government to replace the Taliban should it fall as a result of American military strikes or internal unrest.

"We're not into nation building," he said. "We're focused on justice."

How it would be possible in practice for the United States to engage in military action that might lead to the downfall of a government without taking any interest in its successor was not immediately clear.

In his presidential campaign, Mr. Bush and his advisers dismissed the military and foreign policy goals of the Clinton administration as "nation building," which they characterized as a doomed effort to impose American values on countries unequipped to handle them.

Nevertheless, the question of what happens in Afghanistan if the Taliban falls or if military action drives millions of refugees toward the borders is urgent. But Mr. Bush made it clear today that he was not yet prepared to address that in detail.

His remarks echoed those of his national security adviser, Condoleezza Rice, who said on Sunday that the Afghans would be "better off" without the Taliban government.

American military and intelligence officers in the region are working with opposition groups to prepare military action, gather intelligence and pressure the Taliban.

Pentagon planners said today that one concern was that a huge refugee flow out of Afghanistan could undermine the shaky government of neighboring Pakistan, which Washington expects will play a crucial role in any direct action against the Taliban.

Pakistan's situation was among the themes of closed briefings for House and Senate members by Secretary of State Colin L. Powell and Defense Secretary Donald H. Rumsfeld this afternoon. Some members who attended said the administration was clearly grappling with the unintended consequences of the military options under consideration.

In a public briefing at the Pentagon, Mr. Rumsfeld picked up those themes and described an impoverished Afghanistan riven by political, military and ethnic rivalries that could undermine the Taliban from within. But he also noted America's responsibility to help ease the suffering of its civilians.

"There are many Afghan people who are being starved, who are fleeing for their lives," Mr. Rumsfeld said, "and it's just a terrible shame, and we have to do everything possible we can from a humanitarian standpoint to see that their lives are made better than they currently are by the Taliban government and by the circumstance they're living in."

Not all Taliban members agree with its senior leadership's actions for "creating a hospitable environment for Al Qaeda," he said. Among the opposition, the Northern Alliance is engaged in an armed struggle to unseat the Taliban, the secretary said, and tribes in the south might be coaxed into cooperation.

Much of Mr. Rumsfeld's briefing was devoted to describing the complicated campaign against terrorism, a war without a formal opening or ending, and one that will not be without American casualties. The campaign against terrorists who threaten America "will not be an antiseptic war," Mr. Rumsfeld said.

The Pentagon has increased the number of warships and planes in easy striking distance of Afghanistan. But Mr. Rumsfeld's comments and those of other senior officials made it clear that the opening stage of the campaign was highly unlikely to be a decisive bombing or missile attack on Afghan targets or a major ground operation. The emphasis is coordinating strikes with other efforts to squeeze operations, especially with law enforcement to crack terrorist cells and with the drive on Mr. bin Laden's financial network.

The Pentagon also announced today the call-up of additional Reserve and National Guard troops, bringing to 14,000 the number mobilized since the attacks. Mr. Rumsfeld has been authorized to activate up to 50,000 guard and reserve troops.

Also today, Prime Minister Junichiro Koizumi of Japan pledged his country's support in the fight against terrorism, in contrast to Japan's neutrality in the Persian Gulf war. The Japanese offered intelligence and logistical assistance to the United States, a step that lets Japan stay within the bounds of its postwar constitutional ban on collective self-defense. They also pledged $40 million in aid to Pakistan.

The White House announced that although Mr. Bush will attend the Asia Pacific Economic Cooperation Leaders Meeting in Shanghai from Oct. 20 to Oct. 21, as planned, he will not visit Tokyo, Seoul and Beijing.