WASHINGTON - Lawmakers this week will press the U.S. military's top uniformed officers for the first time on whether they think repealing "don't ask, don't tell" makes sense or would be too disruptive.

The testimony from each of the service chiefs on Capitol Hill will be crucial to the debate in Congress on whether to repeal the 17-year-old law, which bans gays from serving openly in the military.

President Barack Obama says the policy unfairly punishes patriots who want to serve their country. Defence Secretary Robert Gates agrees and has begun a yearlong study on how to mitigate the impact of lifting the ban.

Providing much-needed political cover is the chairman of the Joint Chiefs of Staff, Adm. Mike Mullen, who has said he thinks the law unfairly forces gay troops to compromise their integrity by lying about who they are.

But lawmakers, who are divided on whether to end the ban, say they want to hear from the service chiefs. They are the ones who would be in charge of putting any changes in place and responding to any fallout.

"The armed forces have always placed military effectiveness above individual needs," said Rep. Gene Taylor, a conservative Democrat from Mississippi who says he is unconvinced that the ban should be lifted.

"This is one of the core concepts that has made the U.S. military one of the most effective combat forces in history," he said.

While Mullen says he believes the ban should be lifted, he has said he can't speak for the service chiefs other than to say they support Gates' yearlong assessment.

The service chiefs are scheduled to testify separately throughout the week, with the Army's Gen. George Casey and the Air Force's Gen. Norton Schwartz going first on Tuesday. Marine Corps Commandant Gen. James Conway, who is said to oppose changes to the policy, will testify Wednesday.