The Bush administration has yet to make the case for sending U.S. troops into Liberia either to Congress or to the American people, says the Republican chairman of the Senate Armed Services Committee (search).

Friday's comments by Sen. John Warner (search) came hours after the Pentagon canceled plans to give his committee a private briefing on Liberia (search). Warner said that in his 25 years in the Senate, "I'm not sure I know of a precedent of that type of abrupt cancellation."

"If I were asked today what should be done, I would simply say I do not have the facts to make an informed decision," said Warner of Virginia. "I hope in the executive branch there are those who do have sufficient facts to make an informed decision."

President Bush has ordered a three-ship amphibious group carrying 2,000 Marines to position itself off Liberia. Two of the three arrived Monday for what Bush has said would be limited U.S. involvement.

Liberian President Charles Taylor is under intense attack by rebels and is facing international pressure to resign and leave the country. The first group of West African peacekeepers arrived Monday.  Taylor now says he plans to leave by Aug. 11.

It is not clear what role U.S. forces would play in Liberia. Many lawmakers say U.S. forces are already stretched too thin with military missions in Iraq, Afghanistan and worldwide in the campaign on terror. They question whether Liberia poses too great a risk to American soldiers.

Warner has been a loyal supporter of Bush and a strong defender of his handling of the Iraq war. Any doubts he raises about the Liberia mission could influence other Republican lawmakers.

Warner said he recognizes "the dimension of human suffering today and the potential for even greater human suffering tomorrow" in Liberia, where aid workers say more than 1,000 civilians have been killed since early June.

But he questioned whether it meets the standard for U.S. military involvement after the Vietnam War, such the "Powell doctrine" set forth by Secretary of State Colin Powell when he was chairman of the Joint Chiefs of Staff during the Persian Gulf War.

Those standards say military force should be in response to a clear threat, have public support, involve the use of overwhelming force and include a clear exit strategy.

"Will the framework be the guideline, or are we somehow going to make a departure?" Warner said.

He also asked whether the American people have been prepared to accept casualties in Liberia. "I fear, in my judgment, that has not been done," he said.

Warner noted that U.S. involvement could begin while Congress is on its August recess, and he hopes lawmakers would be consulted when they return. He said it would be up to the administration and Senate leaders to decide whether Congress should be asked for a formal resolution of support.

Warner will visit Africa during the recess as part of a delegation led by Senate Majority Leader Bill Frist, R-Tenn.