The measure, an amendment to a defence spending bill being debated in Congress this week, aims to prevent "cruel, inhuman, or degrading treatment or punishment of persons under custody or control of the United States government".

The bill's main sponsor, Republican Senator John McCain, had predicted easy passage on Wednesday.

"The effect would be that the men and women who are doing the interrogations would feel comfortable in knowing that they have exact instructions as to what they can and cannot do," said McCain, a decorated war veteran who spent more than five years in a Vietnam prison camp.

US message

"It would send a message throughout the world where America's image is suffering that we will not condone any practice that is cruel or inhumane," McCain said earlier in the day.

The bill was a response to the damaging 2004 prison abuse scandal that erupted following the publication of pictures that showed US military personnel humiliating and abusing inmates at Iraq's Abu Ghraib prison.

"It would send a message throughout the world where America's image is suffering that we will not condone any practice that is cruel or inhumane"

John McCain,
Republican Senator

The photos showed detainees piled up naked on the floor in front of US soldiers, cowering in front of snarling military dogs, chained to beds in stress positions, and forced to stand naked in front of female guards.

The measure states that "no person in the custody or under the effective control of the Department of Defence or under detention in a Department of Defence facility shall be subject to any treatment or technique of interrogation not authorised by and listed in the United States Army Field manual on intelligence interrogation".

"The image of the United States was very badly harmed by the pictures of prisoner abuse," said McCain. "We have to send a message to the world that we will not ever allow such kind of treatment to be repeated."

Bush opposition

US President George Bush, however, has vowed to veto the measure.

"There still seems to be significant opposition from the White House and the Department of Defence," McCain said. "We'll have to keep working it."

But he added that members of the US military have contacted him expressing support for the measure.

Misbehaviour

McCain said: "We're hearing from men and women in the military all the time. They don't like to see their image tarnished by allegations of misbehaviour such as Abu Ghraib and other instances."

Colin Powell fully supported the
amendment, McCain said

From the floor of the Senate, he also read a letter of support from former secretary of state Colin Powell, who once served as the chairman of the Joints Chiefs of Staff at the Pentagon.

"I fully support this amendment," McCain said, reading from Powell's letter, dated 5 October.

"The world will note that America is making a clear statement with respect to the expected future behavior of our soldiers," the former secretary of state was quoted as writing.

"Such a reaction will help deal with the terrible public diplomacy crisis created by Abu Ghraib," said Powell.