The number of hunger-striking Guantánamo detainees being force fed by military medical teams has jumped to 41 and now makes up a quarter of the camp's prisoner population.

The new figures reveal a continued deterioration of conditions at the camp as a hunger strike by many of Guantánamo's 166 detainees has entered a fourth month amid little sign of it ending.

In all, 103 inmates are now on hunger strike, with four being hospitalised. The number of inmates being force fed was 31 on the eve of a speech by President Obama last month in which he promised to work towards closing the base and allow the release of many of the 86 prisoners held there who have been cleared for transfer.

Yet, despite the warm reception to that speech, the hunger strike at the base continue to increase in scope as more detainees are ending up being force fed through tubes put up their noses and into their stomachs.

"The hunger strike grows for two reasons: the military's refusal to negotiate with the men in a productive way and because the president has taken no action in spite of his words," said Carlos Warner, a lawyer who represents several of the detainees on strike.

The protest began in February after a search of cells by guards turned up hidden contraband among the prisoners but also led to accusations of heavy-handedness. The strike rapidly grew, creating global headlines and focusing a spotlight on the inmates' plight where many of them – including those cleared for release – have been detained without trial for more than a decade.

It also highlighted Obama's failure to fulfil a 2008 campaign promise to close the base. In his May speech Obama repeated that vow and described Guantánamo as a moral problem for America that needed to be solved. He lifted a self-imposed US ban on prisoner transfers to Yemen – which is keeping 56 cleared Yemenis from being sent back to their home country – and said he would appoint a top official to handle releases. However, so far no one has been named to the job, nor has any detainee been transferred.

Lawyers said the tempo of the strike was not changing. "We have had several phone calls calls this week with our clients and there is no sign of the strike ending. The men are suffering. One client reported he has lost 70lbs during his peaceful hunger strike," Warner said.

A military spokesman, however, insisted that there was no threat to the detainee population's health, despite so many inmates being force fed by a military medical team flown out to the island of Cuba specially for the task.

"The detainees health is constantly monitored by numerous specialists, so in that regard, no, we are not concerned about the number [of detainees being force fed] per se, as we will continue to provide safe and secure care and custody of the detainees as they continue their protest," said Col Samuel House in a statement to the Guardian.

Detainees have described to their lawyers in phone calls and letters a hard regime at the base, with confiscations of many basic items, like toothbrushes. They have also accused guards of being rough with them whilst force-feeding and using the threat of intimate body searches as a way of putting detainees off having phone calls with their lawyers. The military has strongly denied those allegations.

Omar Farah, a lawyer with the Center for Constitutional Rights, predicted the strike was likely to continue unless action happened soon. "Conditions at Guantanamo are appalling … the image of 41 men strapped to restraint chairs, gagging from the pain of nasal-tube feedings, ought to compel the president to resume transfers. Until that happens, we will continue to hear chilling news from Guantánamo," he said.

But the issue of force-feeding has become a controversial one. It has been condemned by human rights activists as a violation of prisoner rights and international medical experts have described it as inhumane.

Cori Crider, strategic director at legal charity Reprieve, who represent several hunger-strikers, said the process was a brutal one. "My clients are protesting their indefinite detention using the only tool at their disposal: a hunger strike. They don't want to be fed. And yet detainees are being brutally force-fed," she said.