Secretary Gates says he has asked some questions about the request, and commanders are working on providing him the answers. At a news conference Thursday, he indicated he is trying to balance several factors.

"I am concerned about relieving the pressure on our allies to fulfill their commitments," he said. "I am concerned about the implications for the [U.S.] force. And I also am very concerned that we continue to be successful in Afghanistan and that we continue to keep the Taliban on their back foot and that we defeat their efforts to try and come back."

Secretary Gates has been pressing NATO allies to meet the shortfall of more than 7,000 troops in the alliance's Afghanistan battle plan. But in recent months he has acknowledged that the allies may simply not be able to come up with the combination of operational forces and trainers that Afghanistan needs.

The request he is expected to receive Friday appears to be something of a compromise, calling for 3,000 U.S. Marines to be sent to Afghanistan for just seven months. But officials say they will arrive in Afghanistan at a crucial time, as the usual spring fighting season begins and as Afghan forces are expanding and will need more trainers to work with their new units.

U.S. officials say foreign and Afghan troops blunted an expected Taliban offensive last year, and they are determined to do the same this year. Still, violence rose last year, particularly in southern Afghanistan, where many of the extra U.S. troops are expected to be deployed.

In Kabul Thursday, Afghan Foreign Ministry spokesman Sultan Ahmad Baheen welcomed the possible increase in the U.S. deployment.

Baheen said the extra U.S. troops will fill an important need to increase efforts to strengthen and equip Afghan forces so they can take on more of the security burden.

The United States has about 27,000 troops in Afghanistan, of the total of 55,000 foreign forces. About half of the U.S. troops work within the NATO structure, most of them in eastern Afghanistan, and the rest conduct counter-insurgency and counter-terrorism operations throughout the country under U.S. command.