The decision acknowledges Iraq is much more unstable and dangerous than U.S. generals had hoped earlier this year, when they planned to cut the number of troops occupying Iraq to about 115,000.
Since then, violence by Sunni and Shiite Muslim extremists has surged, making April the deadliest month for American troops since the March 2003 invasion of Iraq. Several U.S. allies also decided to pull their forces out, most notably Spain, which had more than 2,300 troops in one of the most volatile areas of south-central Iraq.
Defense Secretary Donald H. Rumsfeld on Tuesday ordered about 10,000 active-duty Army soldiers and Marines to prepare to ship out to Iraq in the next few months. They will help replace 20,000 U.S. troops being kept in Iraq for as long as three months past their one-year tours of duty.
Another 10,000 active-duty troops will be called up to fill out the replacement forces, Rumsfeld said.
In other developments:
The troops coming into Iraq will be more heavily armed than the forces they replace, with more tanks, armored personnel carriers and armored Humvees, said Lt. Gen. Norton Schwartz of the Pentagon's Joint Staff.
"The mission remains essentially the same. It's security and stability," Schwartz told reporters at the Pentagon.
Rumsfeld told a news conference that he approved the 20,000 extra troops at the request of Gen. John Abizaid, top commander of U.S. forces in the Middle East.
In addition, the Army announced that about 37,000 National Guard and Reserve troops are being called to active duty to support three National Guard combat brigades that will be sent to Iraq late this year or early in 2005.
About 5,000 Marines and a contingent of about 5,000 soldiers from the 10th Mountain Division will go this summer to relieve the 1st Armored Division and the 2nd Armored Cavalry Regiment, whose soldiers were due to come home in April but were extended until June.
CBS News Correspondent Elizabeth Palmer reports the Army recognizes that soldiers coming back from Iraq need time to decompress, so it organizes a week of sessions to help them come to grips with disturbing things they've seen. The trouble, though, for some of these soldiers is that the call to go back came just as they were struggling to put Iraq behind them.
The 10th Mountain Division's 2nd Brigade will go to Iraq. The Marine units are the 11th Marine Expeditionary Unit and the 24th MEU.
Pentagon officials had said in recent weeks that they were prepared to replace a portion or all of the 20,000 1st Armored and 2nd Cavalry soldiers who are on extended duty in Iraq if Gen. Abizaid believed they were needed.
Abizaid and his subordinate commanders have used the 2nd Armored Cavalry and 1st Armored to deal with outbreaks of violence in and around the Shiite holy city of Najaf and elsewhere in central Iraq.
The Army and Marine Corps are hard-pressed to find substantial additional troops for Iraq duty. Of the Army's 10 divisions, parts or all of nine are already deployed in Iraq or Afghanistan.
The 10th Mountain Division, which is mainly a light infantry unit, has soldiers in both Iraq and Afghanistan.
The Marine Corps has about 25,000 troops in Iraq, mainly in the western area including the restive city of Fallujah.
The 37,000 National Guard and Reserve troops who are being alerted for Iraq duty will provide support for the three National Guard combat brigades that were notified earlier this year that they will be going to Iraq for one-year tours late this year or early in 2005.