President Barack Obama has vowed a renewed push to close the military jail at Guantanamo Bay because he says it is damaging US interests.

Obama's announcement on Tuesday came as extra medical staff were sent to the centre to help address a hunger strike that spread to nearly two-thirds of the detainees.

He told a news conference that he had asked a team of officials to review the issue and would make another appeal to Congress to shut down the prison holding "terror" suspects in Cuba.

Obama, who ordered the detention centre closed upon taking office but was thwarted repeatedly by Congress, said he was unsurprised that there were problems at the centre, where 100 of 166 inmates were now on hunger strike.

"It is critical for us to understand that Guantanamo is not necessary to keep America safe," he said.

"It is expensive. It is inefficient. It hurts us in terms of our international standing. It lessens co-operation with our allies on counter-terrorism efforts. It is a recruiting tool for extremists."

Medical reinforcements

Obama says the jail is costly, inefficient, lessons global co-operation and is used as a recruiting tool by extremists [EPA]

About 40 US Navy medical personnel arrived over the weekend, Lieutenant Colonel Samuel House, a military spokesman at Guantanamo, said on Monday.

"The influx of personnel was planned several weeks ago as increasing numbers of detainees chose to protest their detention," he said.

With the strike now entering its 12th week, Obama has faced fresh calls to honour his promise to close the prison at the US base in Cuba, which holds 166 people captured as part of the "War on Terror."

House said 21 of the inmates on strike were receiving feeding through nasal tubes and five were in hospital. He did not specify in the written statement whether any were in a life-threatening condition.

Lawyers for the detainees said that about 130 inmates were observing the hunger strike, more than officially acknowledged.

The rapidly growing protest movement began on February 6, when inmates claimed prison officials searched Qurans in a way they considered blasphemous, their lawyers said.

Officials denied any mishandling of Islam's holy book, but the strike has now turned into a larger protest by prisoners against their indefinite incarceration without charge or trial over the past 11 years.

Calls for closure

Former chief prosecutor at Guantanamo, Air Force colonel Morris Davis, is among an increasing number of critics who have called for the centre's closure.

"Unless President Obama acts soon, I believe it is likely one or more of the detainees will die," Davis said.

Senator Dianne Feinstein, a Democrat from California, wrote a letter to Obama asking the administration to renew its efforts to transfer out the 86 detainees who were cleared for such a move by US military authorities.

She called for the reassessment of the "security situation on the ground in Yemen, because it is my understanding that 56 of the 86 detainees cleared for transfer are Yemeni."

Obama imposed a moratorium on repatriating Yemenis held at Guantanamo in 2009 after a plot to blow up an airliner on Christmas Day was traced back to al-Qaeda's Yemeni franchise.

Rights groups, which have long branded the prison as a legal "black hole," welcomed Obama's remarks but said he could do more.

A large group of politicians, led by Republicans, have said that the jail should stay open and that the detainees were too dangerous to be held on the US mainland.

'No alternative plan'

Protests were held in Yemen last month over 86 Yemenis languishing in the US detention centre [EPA]

"He has offered no alternative plan regarding the detainees there, nor a plan for future terrorist captures," said Buck McKeon, the Republican chairman of the House Armed Services Committee, of the president in a written statement.

White House spokeswoman Caitlin Hayden said early on Wednesday that Obama was examining options aimed at reducing the number of inmates and moving towards the ultimate closure of the jail.

She said Obama could name a new senior State Department officer to refocus on repatriating detainees or transferring them to third countries. That post has been vacant since January.

"We will also work to fully implement the Periodic Review Board process, which we acknowledge has not moved forward

quickly enough," she said.

This is a system of parole-style hearings the Obama administration set up but which have left many inmates frustrated over the slow handling of their cases.