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The Pentagon has begun surveying US troops about their attitudes and behaviour toward gay comrades.

The survey of roughly 400,000 service members comes as the US military weighs ending a ban on gays serving openly.

It asks troops if the presence of people thought to be gay has affected morale and combat performance.

It also asks if they have showered with gay comrades and whether they would attend social functions if soldiers' same-sex partners were invited.

The confidential survey is part of a Pentagon effort to determine the effect on the US military if the so-called "Don't Ask, Don't Tell" policy barring gays from serving openly is repealed.

The Pentagon has not publicly released the survey but it was obtained by the Palm Center, a research organisation affiliated with the University of California at Santa Barbara.

President Barack Obama and Adm Mike Mullen, chairman of the joint chiefs of staff, have called for an end to the ban, which was enacted in 1993.

The US Congress is weighing legislation that would repeal the ban if the Pentagon determines it would not hinder military effectiveness.

Since 1994, more than 13,500 US servicemen and women have been discharged under the policy.

Britain, Israel, Canada, Australia, France and at least 20 other nations allow gays to serve openly.

Among other questions, the survey asks what service-members would do if the policy is repealed and they are assigned to use an "open bay" shower with a gay comrade.

It also asks how they would react if they are assigned to on-base housing with a comrade who was living with a same-sex partner.

Options include, "I would get to know them like any other neighbours" and, "I would probably move off base".