The Pentagon has ordered an inquiry into alleged abuses at Guantanamo Bay after reports that camp guards boasted of beating and mistreating detainees.
Hundreds of suspects are being held without charge at the camp
A marine sergeant who visited the camp has said she understood "striking detainees was a common practice".
The sergeant's sworn statement said she had overheard a guard describe slamming a detainee's head into a cell door.
The US has meanwhile rejected a call by British Foreign Secretary Margaret Beckett to close Guantanamo Bay.
Ms Beckett is the highest ranking British official to attack the US over the camp, where hundreds of "war on terror" suspects are being held without charge.
She said the US detention camp did as much to radicalise extremists as it did to promote security.
"The continuing detention without fair trial of prisoners is unacceptable in terms of human rights, but it is also ineffective in terms of counter-terrorism."
But a US spokesman said the camp was needed to house "some very dangerous people", including those who were behind the 9/11 attacks.
The Pentagon's inspector general said the US military's Southern Command, which oversees the Guantanamo Bay camp, had been ordered to investigate complaints of alleged mistreatment.
Beatings 'common practice'
Military lawyers who represent detainees at the camp have filed an affidavit that describes guards boasting of abusing prisoners.
Marine Sgt Heather Cerveny, who went to the base three weeks ago as a legal aide to a military lawyer, said five navy guards described in detail how they beat up detainees.
"The one sailor specifically said 'I took the detainee by the head and smashed his head into the cell door'," she said in the affidavit.
Human rights groups have criticised the camp since it opened in 2002
"From the whole conversation, I understood that striking detainees was a common practice," the sergeant wrote.
"Everyone in the group laughed at the others' stories of beating detainees."
The sergeant also reported that some guards claimed they denied detainees privileges purely to annoy them.
The BBC's James Westhead in Washington says the allegations are significant because they come from a serving member of the US military.
Separately, the Red Cross said on Friday that it had met top terror suspects at the camp.
The US has said it recently transferred the alleged mastermind of the 9/11 attacks, Khalid Sheikh Mohammed, and other al-Qaeda suspects to the camp.
They were believed to have been held before that in secret CIA-run jails.
Some 450 terror suspects are thought to be detained at the camp.