There are credible allegations that Guantanamo hunger strikers are being force-fed in a cruel manner, the UN special rapporteur on torture has said.
The US says inmates are monitored by medical personnel
Manfred Nowak's comments came after it emerged that the number of detainees refusing food at the prison camp had more than doubled since 25 December.
Some 84 inmates are now refusing food, according to the US military.
But a Pentagon official said there was no evidence that they had been treated in an inappropriate way.
Mr Nowak has not been to Guantanamo, and turned down an invitation to the camp because the US refused to give him unrestricted access to the detainees.
He told the BBC that he had received reports that some hunger strikers had had thick pipes inserted through the nose and forced down into the stomach.
This was allegedly done roughly, sometimes by prison guards rather than doctors. As a result, some prisoners had reported bleeding and vomiting he said.
"If these allegations are true then this definitely amounts to an additional cruel treatment," Mr Nowak said.
The allegations were rejected by Pentagon spokesman Lieutenant Colonel Brian Maker.
"To suppose that these people are being left bleeding - I know of no instance of that, there's been no reports of that, there's been no credible evidence produced by any investigation of that fact," he told the BBC.
All those receiving what he called "internal nutrition" were being monitored by trained medical personnel, Lt Col Maker said.
The US military defines a hunger strike as missing nine consecutive meals.
Lawyers for some of the detainees have said the hunger strikers are protesting against their continued detention without trial and against the conditions in which they are being held, he adds.
About 500 prisoners remain at Guantanamo, many of them captured in Afghanistan. Some have been held for nearly four years without charge.
Human rights campaigners have expressed growing concern about the treatment of inmates at Guantanamo.
The Bush administration has denied allegations of abuse at Guantanamo, insisting it does not torture prisoners.