Updated

To help in quelling rising tension in Fallujah and Najaf, the Pentagon will extend the tour of duty of about 20,000 U.S. troops, requiring them to spend up to three more months in Iraq.

"We regret having to extend those individuals," Defense Secretary Donald Rumsfeld announced Thursday, adding "but the country is at war and we need to do what is necessary to succeed."

Approximately 14,500 soldiers of the 1st Armored Division (search), which is based in Germany, plus about 3,200 support troops and about 2,800 soldiers of the 2nd Armored Cavalry Regiment (search) from Fort Polk, La., have been told that they will remain in Iraq for another three months instead of coming home this month, defense officials told The Associated Press before Rumsfeld made his announcement.

"What they're doing is important, it's noble work and in the end it will be successful," Rumsfeld told reporters.

The normal U.S. military force in Iraq is around 115,000 but because of troop rotations, around 137,000 U.S. military personnel are in the country, Rumsfeld said. What Gen. John Abizaid (search), the overall commander of U.S. forces in Iraq, requested was the chance to keep the force level at the higher number for a few months.

"We have been and are using emergency powers that were granted by Congress to increase the overall numbers," Rumsfeld said.

Marine Corps. Gen. Peter Pace (search), vice chairman of the Joint Chiefs of Staff, said keeping some troops in Iraq longer would not damage U.S. abilities elsewhere.

"We have the capacity with 2.4 million individuals available to us ... to handle this ongoing war and anything I can think of that's on the horizon," Pace said.

Pressed by reporters to answer whether or not increasing the tour of duty for some in Iraq was an acknowledgement that the job was harder than expected, Rumsfeld said: "If a year ago you had asked me to describe where you would be on April 15, 2004, in Iraq, how might you describe it ... I would not have described it precisely as we are now."

"I certainly would not have estimated the number of individuals lost that we have had lost in the last week," Rumsfeld said.

April has become the deadliest month for U.S. forces in Iraq since they set foot in the country in March 2003. The number of wounded also has skyrocketed.

Rumsfeld said last week that he would approve Iraq tour extensions if commanders believed it was necessary to maintain enough combat power to deal with the recent escalation of attacks by insurgents.

In addition, about 3,000 soldiers in a number of transportation and other support units based in Kuwait will be extended beyond one year, an official said Thursday. Many of them are in the National Guard or Reserve. They are deemed critical to resupplying the troops based in Iraq.

At a Baghdad news conference Thursday, Gen. Richard Myers (search), chairman of the Joint Chiefs of Staff, was asked whether the troop extensions indicate plans for large-scale offensive operations. He did not answer directly, saying the move was deemed necessary given "extremist and terrorist acts that must be dealt with."

Myers said it has yet to be determined how long the added combat power will be kept in Iraq.

"It will depend on events here on the ground," he said. "But I think what it shows is our resolve to see this situation through." Myers was in Baghdad for talks with U.S. and coalition commanders and to meet with L. Paul Bremer, Iraq's U.S. administrator.

The advantage of keeping soldiers of the 1st Armored and the 2nd Armored Cavalry in Iraq for an extra three months — rather than bringing in an equivalent number from elsewhere — is that these soldiers have unmatched combat experience in Iraq and familiarity with insurgents' tactics.

The Army is so stretched by its commitments in Iraq, Afghanistan, the Balkans and elsewhere that it has few, if any, forces immediately available to substitute in Iraq for the 1st Armored or 2nd Armored Cavalry.

Also, these units have been heavily involved in one of the most important U.S. military missions there: training thousands of Iraqi security forces. Those Iraqi army and civil defense corps members are central to the Pentagon's plan for eventually turning over military control to the Iraqis and pulling out U.S. troops.

Fort Polk (search), the Army base in Louisiana that is home to the 2nd Armored Cavalry, issued a news release last week quoting the regiment's commander, Col. Bradley W. May, as saying "elements" of his unit "will remain in theater longer than initially announced." He did not say how many soldiers were affected.

The 2nd Squadron of the 2nd Armored Cavalry recently returned home to Louisiana, but the rest of the unit will remain in Iraq. May did not say how much longer his unit would be in Iraq.

Other defense officials said family members were told the soldiers probably would be back at Fort Polk in about four months. They likely will be in Iraq an extra three months, then take a month to redeploy.

The 1st Armored and the 2nd Armored Cavalry are part of a contingent of about 135,000 U.S. soldiers who were being replaced this spring by a fresh group of soldiers and Marines. The 101st Airborne, the 4th Infantry Division and other units recently left Iraq, with the arrival of the 1st Infantry Division, a brigade of the 2nd Infantry Division, the 1st Cavalry Division and the 1st Marine Division. Two brigades of the Army National Guard also are newly arrived.

While surely disappointed that his troops must remain longer than planned, the commander of the 2nd Armored Cavalry has told them they should be ready to help finish the job.

"We are being called to end the fight against Muqtada Sadr's Mahdi Army and we will," May said, referring to the militia of the radical Shiite cleric who has incited violence against the U.S.-led occupation forces in southern cities including Najaf.

Still, the change of plans is bound to take a psychological toll. In a letter to his troops in January, May assured them that their time in Iraq was "fast approaching its conclusion."

The Associated Press contributed to this report.