Military leaders are struggling to choose Army units to stay in Iraq and Afghanistan longer or go there earlier than planned, but five years of war have made fresh troops harder to find.

Faced with a military buildup in Iraq that could drag into next year, Pentagon officials are trying to identify enough units to keep up to 20 brigade combat teams in Iraq. A brigade usually has about 3,500 troops.

The likely result will be extending the deployments of brigades scheduled to come home at the end of the summer, and sending others earlier than scheduled.

Final decisions — which have not yet been made — would come as Congress is considering ways to force President Bush to wind down the war, despite his vow that he would veto such legislation.

In the freshest indication of the relentless demands for troops in Iraq, Maj. Gen. Benjamin R. Mixon, commander of coalition forces in the north, told reporters Friday that his troops have picked up the pace of their attacks on the enemy in Diyala province, northeast of Baghdad.

"Could I use more forces? No question about it," Mixon said, adding that he had asked for more.

The top U.S. military commander in Iraq, Gen. David Petraeus, said a day earlier that it was likely that additional U.S. forces will be shifted to areas outside the capital where militants are regrouping, including Diyala. The region has become an increasingly important staging ground for militant groups, including al-Qaida in Iraq.

"There have been about 30 percent more offensive actions and attacks. Many of those are initiated by us; some are initiated by them," Petraeus said from a military base outside of Tikrit. "I am cautiously optimistic that in the next 30 to 60 days that we're going to see some significant differences in the security situation in Diyala."

If not, he said, he'll go back and ask for still more support.

Petraeus said Thursday that the U.S. buildup in Iraq would need to be sustained "for some time well beyond the summer" to garner the needed results.

Maintaining increased troop levels, said military officials, will require troops to return for what could be their second or third tours in Iraq or Afghanistan, and force military leaders to juggle the schedules to give soldiers a full 12 months at home before returning to battle.

The officials would speak only on condition of anonymity, because no final decisions have been made and no formal requests for the forces have come from commanders in Iraq. But they said it is beginning to appear likely that Petraeus will ask to maintain much of the buildup at least through the end of the year, and possibly into 2008.

One official said planners are scrambling to figure out what combination of units and schedules can be fashioned that could give Petraeus what he wants and have the least negative impact on the troops.

The complex scheduling must identify which units would have been home for 12 months and be trained and ready to go, plus whether the needed equipment would be available and what impact a schedule change has on other plans for the equipment or troops months down the road.

Combat troops, meanwhile, are coming to realize that the Pentagon can't fulfill its commitment to give soldiers two years at home for every year they spend deployed.

At Fort Drum, N.Y., the 1st Brigade of the 10th Mountain Division is already training for a return to Iraq this summer. The brigade, which spent a year in Iraq and got home last summer, is not yet on any official list of units scheduled to deploy, but it's likely to go in late summer.

"It's prudent planning for us to be prepared to go back in a year," said Fort Drum spokesman Ben Abel.

Military officials also acknowledge that units scheduled to come home later this summer — such as the 3rd Brigade, 25th Infantry Division — could be forced to extend their tours by up to 120 days to maintain the Baghdad security buildup.

Initially, the Bush plan called for sending 21,500 extra U.S. combat troops to Iraq — mainly to Baghdad — with the last of five brigades arriving by June. So far two of the brigades have arrived in Iraq. The latest estimates indicate that up 7,000 support troops may also be needed, including more than 2,000 military police.