A 40-strong medical back-up team has arrived at Guantánamo Bay, as the number of inmates taking part in a hunger strike continues to rise, the US military has confirmed. By Monday, 100 detainees were refusing food, with 21 having been approved for force-feeding.

Authorities said that the "influx" of medical reinforcements had been weeks in the planning. But the news will fuel speculation that the condition of hunger-striking prisoners at Guantánamo Bay is deteriorating. Shaker Aamer, the last British resident being kept at the centre, told his lawyer earlier this month that authorities will soon see fatalities as a result of the current action.

"I cannot give you numbers and names, but people are dying here," said Aamer, who is refusing food.

The action is a protest against conditions at the centre, as well as the indefinite nature of the remaining prisoners' confinement. Aamer has been cleared for release twice, but is still behind bars after 11 years. He has never been charged or faced trial but the US refuses to allow him to return to the UK, despite official protests by the British government.

Of the 166 detainees left at Guantánamo, almost two-thirds are on hunger-strike. Five of those approved by guards to be subjected to force-feeding are in hospital.

Increased media attention to the plight of prisoners at Guantánamo Bay has led to renewed calls for President Barack Obama to close the camp. In the face of pressure from Congress, Obama dropped a 2008 campaign pledge to close the camp.

The current hunger strike is believed to have begun on 6 February and initially involved a minority of detainees. But the number taking part has steadily increased. Two weeks ago, guards attempted to break the resolve of those refusing food by moving detainees from communal areas and placing them in single cells, where they could be monitored more closely. That action led to violent clashes in which US troops fired four "less-than-lethal" rounds on inmates.

US authorities said on Monday that the decision to bring in a back-up medical team was made as increasing numbers of inmates began to refuse food. "We will not allow a detainee to stave themselves to death and we will continue to treat each person humanely," a Guantánamo Bay spokesman, Lt Col Samuel House, said.

He added: "Detainees have the right to peacefully protest, but we have the responsibility to ensure that they conduct their protest safely and humanely."