The US military is sending additional medical personnel to the Guantanamo prison camp, where more than half the captives have joined a hunger strike to protest their open-ended detention.

Reinforcements numbering fewer than 40 will arrive by the end of April, a spokesman for the detention centre said.

The new arrivals would include a doctor, nurses and medics, who will supplement the 100 medical personnel already on duty.

The military counted 84 of the 166 prisoners as hunger strikers today and was force-feeding 16 of them liquid meals through tubes inserted in their noses and down into their stomachs.

Six were hospitalised for observation.

Hunger strikes have occurred at Guantanamo since shortly after the US began detaining suspected al-Qaeda and Taliban captives there in January 2002.

The current hunger strike began in early February, after guards seized photos and other belongings during a cell search.

Prisoners said the guards had also mistreated their Korans during the search, which the US military denies.

The military has declined to say what prompted the cell searches but similar searches have been conducted in the past.

Though the cell search was the immediate trigger, military officials and lawyers for the prisoners have said the protest generally reflects frustration with the failure to resolve the prisoners' fate.

Most have been held for more than a decade without charge or trial and Congress has blocked Obama administration efforts to close the camp.

Forty-three prisoners had joined the hunger strike by 13 April, when guards in riot gear swept through a communal prison and forced the detainees into one-man cells where they could be better monitored.

Camp officials said the detainees had covered the security cameras and windows, blocking guards' view.

The number refusing meals has grown steadily since then.