Restraint chair used to force-feed detainees on hunger strike at the hospital in Camp Delta, part of the U.S. military prison at Guantánamo Bay, Cuba. Joe Raedle/Getty Images

A Yemeni Guantánamo prisoner who was cleared for release four years ago claims 17 people held at the detention facility have been waging a hunger strike and are being subjected to brutal force-feedings by medical officers.

In harrowing letters sent to his attorneys at the U.K.-based human rights charity Reprieve and obtained by Al Jazeera, Emad Hassan said the hunger strikers have been “divided into two groups.”

“First there is ‘the long term group’ which consists of [six prisoners] who have spent a long time on hunger strike,” wrote Hassan, who has been on a hunger strike since 2007 and is suing the U.S. government to end his force-feeding. “We are treated completely differently to those in the second group. The second group are the other hunger strikers who are treated awfully.”

Hassan said the nurses and corpsmen who conduct the so-called forced cell extractions and administer the feedings are using nasogastric tubes that are too big, and pushing the liquid nutritional supplement through the tube too quickly, causing the prisoners, including one who Hassan said weighs 80 pounds and has a broken arm, to vomit.

“As I write now, brother 171 is vomiting on the torture chair, having been brought there by the forced cell extraction (FCE) team. The nurse and corpsman have refused to stop the feed or to slow the acceleration of the liquids,” Hassan wrote.

Hassan claims that recent news coverage of the prisoners’ plight has lead medical personnel to adopt a “new strategy” of manipulating the actual number of prisoners on hunger strike.

“If someone refuses to be force-fed and complains, the doctor stops feeding them,” he said. ‘Their weight is fine”, [the doctor] says, ‘and it won’t be necessary for them to be fed.’ Meanwhile, their actual weight is lower than the claimed weight ... When the doctor stops feeding four detainees, he decreases the total number of hunger strikers to 13. When their weight goes down and the doctors have to feed him, he cancels three or four others so he can keep the number of hunger strikers low.”

Hassan has compared military personnel at Guantánamo to Nazis and his situation to Anne Frank’s: