WASHINGTON (AFP) - President Barack Obama is considering appointing a new senior State Department official to work out how to transfer detainees to permit the closure of the war-on-terror prison at Guantanamo Bay.

The White House said on Wednesday that Mr Obama would consider the measure as part of a new effort to close the controversial facility that he promised during a press conference on Tuesday.

But the administration also made clear that it would not be able to achieve the closure of the prison in Cuba, while Congress continues to throw up obstacles.

"There are a number of things that we can do," said White House spokesman Jay Carney. "One of the options available to us that we're examining is reappointing a senior official at the State Department to renew our focus again on repatriating or transferring detainees that we determine can be returned to their home countries or third countries."

Earlier this year, the State Department reassigned Daniel Fried, who was the envoy for closing Guantanamo Bay, and he has so far not been replaced.

With most of the remaining 166 inmates at Guantanamo Bay now on hunger strike, Mr Obama said on Wednesday that the camp was no longer sustainable and was not in America's national security interests.

But he also condemned Congress for thwarting plans to bring inmates to the US mainland and for imposing funding constraints which are making it more difficult to transfer detainees who have been cleared for release abroad.

Mr Obama ordered the closure of Guantanamo Bay within a year when he took office in 2009 and his failure to do so remains a blight on his administration.

"There are things that the president can do administratively, but this will also require congressional agreement," Mr Carney said.

"We will work with Congress to try to persuade them of the overriding national security interest as well as economic interest in closing Guantanamo Bay." Mr Carney said all policy towards Guantanamo Bay would be evaluated, including the current moratorium on returning inmates to Yemen, which represent about half of the camp's population, to their homeland.

The moratorium was imposed after an attempt to down a US airliner in Detroit on Dec 25, 2009, was traced back to al Al-Qaeda franchise in Yemen.

The administration reasons that the chaotic situation in Yemen and limited security capability of the government would mean that the risk of released detainees returning to or joining radical groups is too great.

"We're obviously evaluating this and other aspects of the situation in Guantanamo, but ... the moratorium remains in place," said Carney.

Out of 166 inmates held at the prison at the remote US naval base in southeastern Cuba, 100 are now on hunger strike, according to the latest tally from military officers.

And of those, 21 detainees are being fed through nasal tubes.

Obama has long argued for prosecuting enemy combatants in civilian courts and transferring those cleared of wrongdoing to their home countries.

But a majority of lawmakers, particularly Republicans, have insisted the jail should stay open, that the detainees are too dangerous to hold on the US mainland and that the suspects should only be tried before military tribunals.