Washington - Thirteen detainees are on hunger strike at the US military base in Guantanamo Bay, Cuba and are being force-fed through tubes, the US Navy said on Monday.

"Currently, there are 13 hunger strikers at Guantanamo. Two of the 13 have been on hunger strike since August 2005. Most of the others began their hunger strike in January or February," navy Commander Robert Durand said.

He added the group were being fed via tubes, but were otherwise "in good health".

"The involuntary feeding is not designed to break the hunger strike, it is a medical procedure to deliver the appropriate calories and nutrition necessary for good health," he said.

Durand said the start of the hunger strike had coincided with the trial of Australian terror suspect David Hicks at the remote military base last month which was attended by a large number of journalists.

"As soon as the media left, the number of hunger strikers has been steadily dropping," he added.

He added that launching a hunger strike was "a tactic taught in the al-Qaeda training manual ... and is designed to elicit maximum media attention".

Lawyers for several of the men said however the strike had been prompted by harsh conditions at a new maximum-security complex to which about 160 prisoners out of the 385 held at the base have been moved since December, according to the New York Times.

There have been several hunger strikes at the base since 2002. They reached a peak in September 2005 when 131 prisoners were refusing food. Several months later 84 were still on hunger strike, and by May 2006 the figure stood at 89.

All the hunger strikers are fed by a plastic nasal tube which directs food through the nostrils into the stomach, in what military officials said is the "safest" way to ensure prisoners receive adequate nutrition.

Prisoners however have complained that the procedure is humiliating and painful.

One detainee, Isa al-Murbati, from Bahrain, told his lawyer that about 20 prisoners from Camp 6 had launched a hunger strike in January to protest conditions in the new complex, his lawyer Joshua Colangelo said Monday.

Murbati also went on hunger strike for six months in 2005, which he ended after the army began strapping him to a chair to administer large amounts of liquid food which caused violent diarrhoea while still in the chair.

But Durand denied the prisoners suffered during the procedure to force-feed them.

"Medically, it is identical to the procedure that is performed in countless US hospitals thousands of times per day.

"While it is not painful, a local topical anaesthetic is offered, and a small tube provides the calories and nutrition necessary."

Newly released Pentagon documents show that during earlier hunger strikes, before the use of restraint chairs, some detainees suffered sharp weight losses, the Times said.

A handful of those prisoners lost more than 13,5kg in a matter of weeks, according to the report.