Human rights groups welcomed Obama's recommitment to shutting the prison. But some activists called for action, not just words, and said the president could take some steps on his own without hitting congressional obstacles.
"It's not sustainable - I mean, the notion that we're going to continue to keep over 100 individuals in a no-man's land in perpetuity," Obama said.
Obama lamented the status quo, which has kept most prisoners in detention without trial or charge since the prison was set up at the US Naval Base on Cuba in 2002 to hold foreign terrorism suspects.
A renewed effort to close the camp would mean finding a series of solutions - some of which would likely come up against the same congressional opposition they faced in the past given lawmakers' reluctance to have inmates transferred to the United States.
Obama, who repeatedly pledged to close the camp when he was campaigning for a first term and after he first took office in 2009, put the blame on Congress for his failure to make good on his promise and said he would re-engage with lawmakers on the issue.
While Obama acknowledged an uphill fight and provided few specifics on how to overcome legal and political obstacles, White House spokeswoman Caitlin Hayden later said he was weighing a range of options aimed at reducing the number of inmates and moving toward "ultimate closure."
She said Obama could implement some measures on his own, including naming a new senior State Department officer to refocus on repatriating detainees or transferring them to third countries, a process that has ground to a halt. 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. Obama's comments were his first public remarks about Guantanamo since the hunger strike began in early February.
Military officials have attributed the protest in part to a sense of hopelessness among detainees over their open-ended detention.
Long a subject of international condemnation but low on the list of the American public's policy concerns, Guantanamo has been thrust back in the spotlight by the hunger strike and the military's decision to force-feed prisoners to keep them alive.
The US military has said 21 prisoners are being force-fed liquid meals through tubes inserted in their noses. Forty medical personnel have been sent to reinforce the military's existing teams at Guantanamo to deal with the hunger strike.
Violation of medical ethics
Some inmates have given harrowing accounts of force-feeding, and the practice has been criticized by rights groups and also by the American Medical Association.
On Thursday, the president of the AMA sent a letter to defense secretary Chuck Hagel reiterating the association's position that it is a violation of medical ethics to force-feed mentally competent adults who refuse food and life-saving treatment.
Asked about the force-feeding, Obama defended it, saying "I don't want these individuals to die." On the hunger strike, he said it was "not a surprise to me that we've got problems in Guantanamo."
"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 cooperation with our allies on counter-terrorism efforts. It is a recruitment tool for extremists. It needs to be closed."
Obama said he had asked his advisers to "examine every option that we have administratively" to deal with Guantanamo. It was unclear whether that meant Obama might use executive powers that some legal experts say he has to transfer some detainees.
US senator Patrick Leahy, the Democratic chairman of the Senate Judiciary Committee, backed Obama's effort. "The deteriorating situation at Guantanamo, including the ongoing and expanding hunger strikes by prisoners ... is disturbing and unacceptable," he said.
But Howard McKeon, Republican chairman of the House of Representatives Armed Services Committee, said, "The president faces bipartisan opposition to closing Guantanamo Bay's detention center because he has offered no alternative plan regarding the detainees there, nor a plan for future terrorist captures."
Obama has approved military tribunals to try some of the most dangerous suspects, but only nine of the current prisoners have been charged or convicted of crimes. Of the other inmates, 86 have been cleared for transfer or release, 47 are considered too dangerous to release but are not facing prosecution and 24 are considered eligible for possible prosecution.
US lawmakers, mostly Republicans but including some Democrats, have blocked Obama from transferring Guantanamo prisoners to American jails, saying they would pose a security risk if housed in the United States.
The US government will not send some prisoners back to their homelands because of instability or concerns over mistreatment. Most countries are reluctant to accept them for resettlement when the United States itself will not take them. Obama said ultimately he would need approval from Congress to shutter the facility and acknowledged that would be an uphill struggle, saying, "It's easy to demagogue the issue."
The Center for Constitutional Rights, which has long campaigned to close Guantanamo, said: "We praise the president for reaffirming his commitment to closing the base but take issue with the impression he strives to give that it is largely up to Congress."
It said that if Obama were "really serious" about closing the camp, he could use a "waiver process" to transfer detainees, starting with the 86 men who have already been cleared for release, lift the moratorium on transfers to Yemen and appoint a senior administration official to shepherd the closure.
The United States has not sent prisoners back to Yemen, where 56 of those eligible for release are from, since a foiled plot in 2010 to bomb an American passenger aircraft was hatched my militants in Yemen.
The Guantanamo camp was opened by Republican president George W Bush, to hold foreign terrorism suspects captured overseas after the Sept. 11 attacks on the United States in 2001.
Obama failed to meet his promise to close the prison within a year of taking office in early 2009 and it has become an enduring symbol of widely condemned US interrogation and detention practices during the Bush era.
An independent US task force issued a report on April 16 calling indefinite detention of prisoners at Guantanamo "abhorrent and intolerable." It called for the camp to be closed by the end of 2014 when NATO's combat mission in Afghanistan is due to end and most US troops will leave the country.
The US military on Monday counted 100 prisoners as hunger strikers. Five of those being force-fed have been hospitalised for observation but did not have life-threatening conditions, a spokesman for the detention camp, Army lieutenant Colonel Samuel House, said on Tuesday.
Hunger strikes have occurred at Guantanamo since soon after it opened. The current protest began in early February, after guards seized photos and other belongings during a cell search. Prisoners said the guards had mistreated their Korans during the search. The US military has denied that.