WASHINGTON — In a sign of possible differences among top military officials, Army and Air Force chiefs voiced concern yesterday about ending a ban on gays serving openly in the armed forces while the country is in the midst of two wars.

Army Gen. George W. Casey and Air Force Gen. Norton A. Schwartz both told Congress they support the Pentagon's plan to spend a year studying a change in the policy allowing gays to serve only if they keep their sexual orientation hidden.

However, both generals were mum about their own views on gays in the military and neither followed the lead of Adm. Michael G. Mullen, the chairman of the Joint Chiefs of Staff, who earlier this month said gays should be allowed to serve openly.

Lawmakers and advocates are carefully watching the congressional testimony, trying to gauge where the various service chiefs stand on the issue of gays in the military as a barometer for the eventual outcome of the issue.

Opponents of the ban on gays, including President Obama and many congressional Democrats, want to quickly overturn it. However, supporters of the ban, including some congressional Republicans, are looking to the military officials for possible support for keeping the policy in place.

Casey and Schwartz carefully followed a middle path outlined in recent months by Defense Secretary Robert M. Gates, who supports Obama's call to end the ban but wants any change to be made slowly and studied carefully.

Obama said in his State of the Union address in January that he wanted the 17-year-old policy known as "don't ask, don't tell" rescinded this year.

Military officials earlier this month said they intend to spend the year on a study to assess the effects of a policy change.

SOME RESERVATIONS

Appearing yesterday before the Senate Armed Services Committee, Casey said he had reservations about the effect of a change on deployed troops.

"I do have serious concerns about the impact of repeal of the law on a force that is fully engaged in two wars and has been at war for 8 1/4 years," Casey said. "We just don't know the impacts on readiness and military effectiveness."

"Exactly," murmured Sen. John McCain, R-Ariz., in response.

Appearing at the same time before the House Armed Services Committee, Schwartz said there were few reliable surveys about what airmen and their families think of the policy. Schwartz acknowledged Obama has stated he wants the current law to be changed, but Schwartz said potential "complications" should be examined first.

"This is not the time to perturb the force that is, at the moment, stretched by demands in Iraq and Afghanistan and elsewhere, without careful deliberation," he said.

The appearances by Schwartz and Casey will be followed today by Adm. Gary Roughead, the chief of naval operations, and Marine Corps Commandant Gen. James Conway, who is said to be an opponent of lifting the ban.

MORE THAN 14,000 BOOTED

As part of the 1993 law creating the "don't ask, don't tell" policy, the Pentagon is required to remove service members accused of being gay or admitting so. More than 14,000 service members have been booted out after being accused of being gay or having said they were.

Sen. Carl M. Levin, the chairman of the Senate Armed Services Committee, may push for a congressional moratorium to halt discharges during the Pentagon's study.

However, McCain, the ranking Republican on the committee, said the moratorium would circumvent the purpose of the study.

On Monday, Army Gen. Raymond T. Odierno, the top commander in Iraq, voiced his support for the Pentagon policy study. But unlike Casey, Odierno voiced personal support for changing the policy.

"My opinion is everyone should be allowed to serve, as long as we're still able to fight our wars and we're able to have forces that are capable of doing whatever we're asked to do," Odierno said.

But in a reflection of the complex military thinking on the issue, Odierno, like Casey, said his primary concern was for troops serving in war.

"We're in two wars right now. So I want to see it done properly," Odierno said.