The overturning of 'don't ask, don't tell' – the policy banning openly homosexual soldiers from serving in the US military – is likely to be long and tortuous, based on the response of military leaders appearing before Congress today.

Both the US Air Force chief of staff, General Norton Schwartz, and his counterpart at the US army, General George Casey, told congressional committees that great care was needed given the committments of US armed forces fighting two wars.

"This not the time to perturb the force … without careful deliberations," Gen Schwartz told the House of Representatives's armed services committee when asked his opinion of the moves to repeal DADT.

Schwartz said "careful deliberation" was needed and is concerned that not enough research through academic studies and surveys of military opinion were being produced.

Schwartz's view was echoed by Casey, saying: ''I do have serious concerns about the impact of a repeal of the law." He concluded: "We just don't know the impact on readiness and military effectiveness."

Casey was questioned by Senator Jack Reed, a Democrat with long military service, if he knew of any problems that US forces had serving alongside with other countries, such as Britain and Canada, which have already removed bans on gay soldiers. Casey replied that he had ''heard nothing from anyone about any conflicts with British or Canadian soldiers or any other countries' soldiers that have already implemented that policy".

The US military leaders were appearing on Capitol Hill to discuss next year's budget, but politicians couldn't resist sniping over the year-long review of "don't ask, don't tell" that the White House announced last month.

Separate simultaneous hearings took place at the Senate and House of Representatives's armed servcies committees, with Casey appearing before the Senate committee and Schwartz appearing before the House committee.

At the Senate hearing, both Casey and the new secretary of the army, John McHugh, said they were opposed to suspending DADT while the defence department's review is underway. Casey said a moratorium "would complicate the whole process'' because of legal issues with the current policy. '"Anything that complicates it more, I would oppose that,'" he said.

Opposition to a moratorium was led by Senator John McCain, with Republicans afraid that a moratorium would be added as a rider to the armed forces budget bill going through congress.

The argument against delaying repeal of DADT was bolstered today by a new study from the Palm Centre, a California university thinktank. It found that armies that allow gays and lesbians to serve openly experienced little or no disruption by implementing the new policy rapidly.

"Decisive action is a must, while slow-rolling implementation carries risks of muddling the process," said Nathaniel Frank, the report's lead author.