Detainees on hunger strike at Guantanamo are "devastated" after their "optimism" was dashed that the military prison would be closed, and are trying to get the issue "back into the media" by starving themselves, the facility's commanding general has said.

General John Kelly, the commanding general responsible for the prison, told a congressional committee in Washington on Wednesday that detainees "had great optimism that Guantanamo would be closed. They were devastated apparently ... when the president backed off, at least (that's) their perception of closing the facility."

The number of detainees on hunger strike has nearly doubled since March 15 to 24.

Eight of them lost enough weight that doctors were force-feeding them liquid nutrients thorough tubes inserted into their noses and down their stomachs, Captain Robert Durand, a spokesman at the detention camp, told Reuters news agency.

Two were hospitalised with dehydration, he said.

Frustration

Kelly attributed the hunger strike to frustration among the 166 Guantanamo prisoners, who were captured in overseas counterterrorism operations after the September 11, 2001, attacks. Nearly all have been held for 11 years without charge, and half have been cleared for transfer or release.

In his testimony, Kelly said he believed the intention of the strikers was to "turn the heat up, get it back in the media."

Upon taking office in 2009, President Barack Obama ordered the detention camp closed within a year, but Congress has blocked administration efforts to shut it down and made it increasingly difficult to resettle Guantanamo prisoners.

Obama did not mention Guantanamo in his January inaugural speech or his February state-of-the-union address, which some of the prisoners watched on television.

In January, the State Department office charged with resettling Guantanamo prisoners was closed.

More than 50 lawyers representing the prisoners sent a letter to Defence Secretary Chuck Hagel last week urging him to help end the current hunger strike.

They said the participants' health had deteriorated alarmingly, and that some had lost up to 14 kg.