WASHINGTON, Aug. 5 — Bush administration officials hailed the arrival of Nigerian peacekeeping forces in Liberia today, but cautioned that conditions were not yet right for American forces to follow suit and that the American military role, if it materialized, would be extremely limited.

The gradual arrival of Nigerian troops in Monrovia, administration officials said, meant that at least one of President Bush's criteria for an eventual decision to introduce American troops was close to being met.

It remained unclear today whether the other main precondition — the departure from Liberia of President Charles G. Taylor — would be fulfilled any time soon. Mr. Taylor has said he would leave office, but not the country, on Monday. American officials said again today that he must do both.

"By definition we are closer today to a possible introduction of American forces," said an administration official. "But no decision on that has been made. The president and the Defense Department have repeatedly said that any role for American forces would be limited, and that still stands."

Other administration officials said that the likely role of American troops would be confined to assisting with logistics, intelligence and communications for African or United Nations forces, and that there would have to be some kind of assurance that if conditions deteriorated, a rescue mission could be mounted quickly.

Those conditions, the official said, were laid down by Defense Secretary Donald H. Rumsfeld, reflecting a general Pentagon uneasiness over making a long-term commitment in Africa while troops are stretched thin in Iraq, Afghanistan and other trouble spots.

Mr. Rumsfeld, speaking at a news conference, noted that the United States military had already begun assisting the Nigerian forces with equipment, assessments of their abilities, transportation and communication.

"That's the sum total of the policy at the moment," he added.

Mr. Rumsfeld was accompanied at the news conference by Gen. Richard B. Myers, the chairman of the Joint Chiefs of Staff, who said that American forces were "very busy" right now in other parts of the world.

Pentagon officials said the first marines from the three American ships off the coast of Liberia to go ashore would be a small group of communications specialists. Their presence on the ground in Liberia would follow that of military survey and assessment teams, which have already spent time in the country.

The Nigerian forces are expected to reach a preliminary level of 3,250 troops, but American officials said that the eventual objective was about 9,000 troops from West African nations.

Mr. Rumsfeld and his aides have cited another reason for opposing a larger American involvement. The political situation is too unstable, they say, for outsiders to enforce a peace among warring factions.

But that worry is being answered, in the view of diplomats at the State Department and the United Nations, by urgent negotiations toward a political arrangement in which a caretaker government would run Liberia while plans are made for power-sharing among various groups.

Among diplomats in the administration, there is some embarrassment that President Bush's promise to be helpful to Liberia, made at the beginning of his trip to Africa last month, has not been followed by an introduction of troops.

Chester A. Crocker, a former head of African policy in the Reagan administration, said today that the Nigerian forces could bring new momentum to a process leading to the deployment of American troops, but that more decisiveness in Washington was needed.

"There is a need to assert leadership in situations like this," said Mr. Crocker, professor of strategic studies at the School of Foreign Service at Georgetown University. "We are seeing some positive movement, but the administration has created expectations and now should step up to the plate."

Secretary of State Colin L. Powell, stung by criticism that he has not been able to persuade Mr. Bush to act more quickly, has defended the administration's approach as anything but indifferent. He has said many times that the best way for Washington to be involved is to work with West African nations under the banner of Ecowas, a regional economic association.

"If the United States was indifferent, we wouldn't be supporting Ecowas," Mr. Powell said last week. "But we have always said from the beginning that it can't just be U.S. support; the U.S. has to support Ecowas. Ecowas is in the lead."

But some impatience is rising. A Western diplomat involved in the issue said today that the cheering in Liberia for the Nigerian force that entered Monrovia on Monday showed that American forces would be welcomed and not have to worry about becoming enmeshed in an endless civil war.

"You saw the picture in the paper today of Liberians carrying a Nigerian peacekeeper on their shoulders," said the diplomat. "That should have been an American peacekeeper they were carrying."

The American plan is for the Nigerians and other West Africans to act as a vanguard force in the first phase. They would be followed by a multinational force in which some form of American participation might take place. In the third phase, the peacekeeping operation would be taken over by the United Nations.

But the timetable for these phases remained uncertain today, and administration officials warned that it could stay that way for days or even weeks to come.