The Army's 3rd Infantry Division, which helped lead the charge to Baghdad at the outset of the war, will return next year and become the first Army division to serve three tours in Iraq.

More than 3 1/2 years into the war, the Army and Marine Corps are straining to keep a steady flow of combat and support forces to Iraq while giving the troops sufficient time between deployments for rest and retraining.

Both services are far short of their goal of providing two years between deployments; the 3rd Brigade of the 3rd Infantry, for example, will have spent barely more than 12 months at home when it returns next year. The same is true for the division's 1st Brigade, which officials have said is scheduled to deploy again in January.

The 3rd Infantry, based at Fort Stewart, Ga., is among several units — totaling 57,000 troops — identified by the Pentagon on Friday for deployment in a fresh rotation of forces starting in January. The announcement does not presume any change in troop levels, nor is any major change expected for at least several months.

The announcement comes as some congressional Democrats, who are poised to take control of the House and Senate, continue to press for a substantial reduction of U.S. troops in Iraq, and a timetable for that drawdown.

There are about 141,000 U.S. troops in Iraq. Gen. John Abizaid, the top U.S. commander for the Middle East, told the Senate Armed Services Committee on Wednesday that he believes troop levels ought to remain fairly steady for the time being, although he said all options are being considered, including a force increase.

Abizaid told the committee that the Army and Marine Corps are not big enough to sustain a substantial increase in Iraq, although he said adding 20,000 troops for a short period was possible.

The Army has managed to keep up its pace of deployments to Iraq and Afghanistan in part by tapping brigades that have been newly created as part of a top-to-bottom reorganization of Army divisions. The 4th Brigade of the 1st Infantry Division, for example, is on the list of units scheduled to deploy to Iraq early next year. That brigade, based at Fort Riley, Kan., was created in recent months and is now at the National Training Center at Fort Irwin, Calif., for an intensive round of final preparations for the Iraq deployment.

Also selected for the next troop rotation is the 173rd Airborne Brigade, based at Vicenza, Italy. Its paratroopers jumped into Iraq at the beginning of the war to open a northern front, and just last February and March the unit's two airborne infantry battalions returned from a full year of combat in Afghanistan.

The newly formed 4th Brigade of the 2nd Infantry Division, from Fort Lewis, Wash., also is part of the next rotation, the Pentagon said, as is the 1st Brigade of the 82nd Airborne Division, from Fort Bragg, N.C.

Combined, the Pentagon announced a combat force totaling 20,000 soldiers, including the 3rd Infantry Division headquarters. In addition, spokesman Bryan Whitman said a support force of 37,000 troops — of whom 27,000 are active duty and 10,000 are National Guard and Reserve — will also deploy in 2007.

They will replace a portion of the current force in Iraq. Additional replacements will be announced next year, Whitman said.

Also, about 1,500 soldiers from the South Carolina Army National Guard's 218th Brigade Combat Team have been told they will deploy to Afghanistan early next year to help train the Afghan army, Whitman said.

The Pentagon also announced Friday that a soldier killed in Baghdad on Tuesday was an Army colonel — the first of that rank to be killed since the war began in March 2003. He was identified as Thomas H. Felts, Sr., of Sandston, Va. William W. Wood, 44, who was killed in Iraq in October 2005, had been approved for promotion to colonel, but at the time of his death he was a lieutenant colonel.

Felts, 45, was assigned to the Command and General Staff College at the School of Advanced Military Studies at Fort Leavenworth, Kan., and was in Baghdad as leader of a team advising the Iraqi army. He died of injuries suffered from a roadside bomb, along with Army Spc. Justin R. Garcia, 26, of Elmhurst, N.Y.

Relatively few U.S. military advisers have been killed in Iraq, although they may face greater dangers if the Pentagon follows through on a plan to expand the number of advisers working alongside Iraqi soldiers and police.

A Marine commander said Friday that he has already begun expanding adviser teams in his area of Iraq's western Anbar province.

Speaking to reporters at the Pentagon by videoconference from his headquarters in Fallujah, Col. Lawrence D. Nicholson said the teams have been doubled in size and he has proposed doubling them yet again.

"We think that is clearly the way ahead," he said, adding that the Iraqis have proven themselves to be good "mimics," emulating the tactics and procedures used by the Americans to be more effective against insurgents.