The US military secretly used a variety of tactics to break the resolve of the Guantánamo Bay hunger strikers, including placing them in solitary confinement if they continued to refuse food, newly declassified interviews with detainees reveal.

One prisoner also said that the last British resident held inside the camp, Shaker Aamer, had been targeted and humiliated by the authorities to the point where it became impossible for the 44-year-old to continue his protest.

The US military recently announced the end of the six-month mass hunger strike among detainees at Guantánamo Bay. But human rights groups argue that such proclamations are disingenuous as at least 16 inmates are still force-fed daily, and two are in hospital.

One detainee, 42-year-old Syrian national Abu Wa'el Dhiab, reported that the Extreme Reaction Force team, the camp's military riot squad, would "storm" Aamer's cell five times a day in an attempt to crush his resolve during the strike.

In letters recounting Aamer's treatment, which have only just been declassified, Wa'el said: "They have deprived him of food, water and medicine. Then the riot squad uses the excuse of giving him water and food and medicine to storm his cell again."

Wa'el, who like Aamer has spent 11 years inside the camp, added: "They took him to the clinic, tore his clothes off and left him with only his underwear for long hours, taunting him."

Another inmate, Samir Mukbel, from Yemen, who has also been cleared for release, alleged that throwing the prisoners into isolation helped break the protest, which lasted more than 200 days and drew such international attention that President Barack Obama reiterated his intention to close the camp.

Ahmed Belbacha, an Algerian detainee who has been cleared for release, corroborated the claim that solitary confinement was used as a punishment for prisoners making political statements. Belbacha, 43, described how the authorities were punishing hunger strikers by confiscating their belongings. "My glasses, legal papers, toothbrush, toothpaste and all my other necessities have been taken."

Testimonies of Belbacha, Mukbel and Aamer are among those featured in an animation narrated by actors David Morrissey and Peter Capaldi depicting life inside Guantánamo Bay. The film also uses testimony from the recently released Nabil Hadjarab, provided by their lawyers at legal charity Reprieve.

Mukbel, 35, added that other tactics were utilised to whittle down the size of the hunger strike. He said the temperature was deliberately manipulated to make conditions inside the camp even more uncomfortable and that during the hunger strike searches of cells were timed to disrupt detainees' sleep.

Cori Crider, a lawyer at Reprieve, said: "The US authorities have, with some glee, announced the hunger strike to be over. What they fail to tell you is the horrific things they did to crush the hunger strikers' spirits, as my clients have described. And yet still there are at least 16 men striking and being brutally force-fed twice a day."

Aamer's lawyers, meanwhile, are concerned over his health. Aamer has refused one visit and three phone calls since August, with his south-London-based family fearful that his treatment has dangerously weakened his health. Meanwhile, it has emerged that prime minister David Cameron has recently written to President Barack Obama in another direct attempt aimed at "securing Aamer's release and return to the UK."

Elsewhere, momentum to secure the release of the 164 detainees appears to be growing with news last week that the Pentagon has appointed a new envoy for the controversial task of finally closing Guantánamo Bay. Officials from several government agencies are expected to re-evaluate previous determinations that some of the men held on the US base in Cuba are too dangerous to release.

The US has not yet said how many of the 164 prisoners now at Guantánamo will be reviewed. More than 80 have already been cleared for release or transfer but are still held either because of restrictions on releases imposed by Congress or because they are from Yemen, which is considered too unstable to take former prisoners.