Washington -- Defense Secretary Donald Rumsfeld announced April 15 that some 20,000 U.S. troops will stay in Iraq for as long as 90 days beyond the scheduled end of their tours of duty.

Rumsfeld told reporters at the Pentagon that Guard and Reserve units will make up about one quarter of those troops. Later, an Army official said some 6,000 Guard and Reservists from 20 states are part of the mix.

Rumsfeld said there are currently around 137,000 U.S. forces in Iraq.

The secretary said the delay in sending the troops home is necessary because "the country is at war and we need to do what is necessary to succeed."

Marine Corps General Peter Pace, vice chairman of the Joint Chiefs of Staff, identified the units involved in the delayed rotation as the 1st Armored Division (based in Germany) and the 2nd Armored Calvary Regiment (based in Louisiana) as well as some helicopter units and combat support engineers.

Pace, who briefed with the secretary, said the length of time that these men and women will continue to be needed in Iraq by their commanders depends upon "the facts on the ground." Pressed further on this point, he said these forces are sustainable in Iraq as long as necessary. He also said the U.S. military is capable of handling this extended deployment concurrently with any other conceivable military operation on the horizon.

Both men provided an update on the situation in Iraq. Rumsfeld said coalition forces had experienced "a tough period of days." He described the situation in southern Iraq as "largely stabilized." He also reported "good cooperation" with moderate Shi'ite leaders.

Asked to describe the situation in Fallujah, Pace said the Marines have halted offensive operations to allow the Iraqi Governing Council and others to go in and access the political situation there. He said the pause in the U.S. offensive in Fallujah does not mean the fighting has stopped. Rumsfeld said military engagements are still occurring.

The secretary was asked if he thought any mistakes have been made in the past year as part of "Operation Iraqi Freedom." He answered the question by saying if he'd been asked a year ago to speculate on what the operation might look like: "I certainly would not have estimated that we would have had the individuals lost that we have had lost in the last week."

Pace said the recent surge in violence in Iraq is being driven by "terrorists" who see the June 30 deadline for turnover of sovereignty approaching rapidly and are petrified by the promise of democracy.

Following the Rumsfeld-Pace briefing, four U.S. Army force planners answered questions about the deployment order. They dismissed the notion that this deployment might cause changes to long-term military plans. One official said planning adjustments are being made constantly because "we are at war."

Asked if U.S. soldiers who are staying on in Iraq are receiving any new equipment, one officer said a number of Stryker armored wheeled vehicles will be assigned to the 1st Armored Division.

In planning for the next rotation of forces in Iraq, another Army official said between 105,000 and 115,000 troops are expected to be in place in the future as greater stability is achieved.