Defense Secretary Donald H. Rumsfeld assured members of Congress on Wednesday that using thousands of National Guard troops to help secure the U.S. border with Mexico will not detract from the troops' ability to perform other missions at home and abroad. He said it would sharpen their skills.

"The up to 6,000 Guardsmen and women proposed for this effort represent less than 2 percent of the total National Guard force of some 400,000, and for the most part they will be deployed during their two- or three-week active duty training period," he told the Senate Appropriations defense subcommittee.

"As such this will not only not adversely affect America's ability to conduct the war on terror or respond to other domestic emergencies, it will actually provide useful real-life training for the members of the National Guard," he added.

Rumsfeld said the White House budget office was still considering whether the beefed-up border operation would require an immediate congressional appropriation of extra funds. The Department of Homeland Security, which is leading the border security program, has yet to tell the Pentagon exactly what missions it wants the Guard to perform, he added.

"Our forces would not be doing law enforcement or standing on the border arresting people or anything like that," he said. Instead they will be providing surveillance, communications and construction help.

Bush's plan has drawn criticism from some members of Congress who say it might overextend a Guard force that already has sent many thousands of troops to Iraq and Afghanistan and deployed others after last year's rugged hurricane season. Analysts have also questioned how much impact the Guard would have along the 2,000 mile-long Mexican border.

Sen. Robert Byrd, D-W.Va., told Rumsfeld that if the Bush administration had not opposed his previous efforts to provide funds to increase the size of the Border Patrol, there would be no need now to call on the National Guard for border duty. Byrd also asked Rumsfeld why the Department of Homeland Security could not contract with private companies to perform the work being assigned to the National Guard.

Rumsfeld said the Guard troops are needed simply as a short-term measure until Homeland Security could expand its capabilities. He said the Guard role would not last longer than two years.

As Rumsfeld finished his opening remarks, a woman standing in the back of the hearing room shouted: "Liar!"

"Whoever that is, would security please remove them," Sen. Ted Stevens, R-Alaska, the subcommittee chairman said.

A uniformed officer approached the woman, who wore a hot pink T-shirt that said "Stop The War Now" and bore the number 2,450, an apparent reference to the number of U.S. military members who have died in Iraq. She was escorted out without a struggle.

Rumsfeld was testifying on President Bush's 2007 defense budget request and the administration's request for an extra $65 billion to cover costs in Iraq and Afghanistan this year. He told the panel that the Pentagon needs the money available by the end of this month to avoid interfering with important military projects, including the training and equipping of security forces in Iraq and Afghanistan.

It was Rumsfeld's first public testimony on Capitol Hill since retired generals issued a series of calls for Rumsfeld to resign earlier this spring. Rumsfeld, with strong public backing by Bush, appears to have weathered that storm.

It also was Rumsfeld's first opportunity to comment on Bush's announcement Monday that, in coordination with governors, he is sending 6,000 National Guard troops to the U.S. southern border.

Although the Guard troops will be operating under state governors' control, the cost will be paid by the federal government, and the administration has not said how much that will be.

Rumsfeld has stressed several times in recent weeks the importance of getting congressional approval for the extra $65 billion in war costs. About half of that is for the cost of conducting the fighting in Afghanistan and Iraq; about $5.9 billion is for developing Iraqi and Afghan security forces, and $1.9 billion is for countering the threat posed by roadside bombs, which are a leading killer of U.S. troops.

The government has so far provided about $368 billion for wars in Iraq and Afghanistan and global operations against terrorism, according to the Congressional Research Service, Congress' nonpartisan research arm. That includes military, foreign aid, reconstruction and veterans spending.