Obama Cautious but Hints at Eventual Action on Syria

President Barack Obama Tuesday warned against a rush to judgment on Syria's use of chemical arms, but said proof of their use would trigger a "rethink" of his reluctance to use military force.

As critics complain that he let Syria cross a U.S. "red line," Obama said Washington believed chemical weapons had been used in the country's vicious civil war but did not know exactly who had fired them.

At a White House news conference, Obama also appeared to set the criteria for a U.S. military intervention as established proof that President Bashar Assad's regime directly ordered the use of chemical weapons.

As more Guantanamo Bay inmates join a hunger strike, Obama also pledged to have another go at closing what he described as the "not sustainable" war-on-terror prison and blamed Congress for the deteriorating situation.

And he praised Russia for its help in investigating the Boston marathon bombings on April 15, which has been blamed on two attackers of Chechen origin, but said "old habits die hard" between U.S. and Russian security services.

Obama faced the press amid rising political pressure over reports by U.S. intelligence that Syrian forces used sarin gas against their foes, despite his previous warnings that deploying chemical weapons would be a "game changer."

"I've got to make sure I've got the facts. That's what the American people would expect."

"If I can establish in a way that not only the United States but also the international community feel confident in the use of chemical weapons by the Assad regime, then that is a game changer," he warned.

"By game changer, I mean we would have to rethink the range of options that are available to us."

"There are options that are available to me that are on the shelf right now that we have not deployed, and that's a spectrum of options," Obama said, saying he had asked the Pentagon for plans, but did not divulge them.

Possible Pentagon options include a risky operation to secure chemical weapons stocks to ensure they do not get into the hands of radical groups or to destroy them, either by an assault team or air strikes.

Obama's foes are also calling on him to directly send U.S. arms to rebels, a step he has so far been reluctant to take, or to establish no-fly zones or safe havens to protect hundreds of thousands of refugees.

One of Obama's first acts as president was an order to close Guantanamo Bay. But four years on, the prison remains open, and a stain on the U.S. image abroad, and is now plagued by multiple hungers strikes by despairing inmates.

"I am going to go back at this," Obama said, warning that the situation at the camp was "not sustainable" and blaming Congress for putting roadblocks in the way of its closure.

"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 recruiting tool for extremists.

"It needs to be closed," Obama said.

"This is a lingering ... problem that is not going to get better. It's going to get worse. It's going to fester," Obama said, adding that the hunger strike should cause Americans to think about how to better prosecute, confine or repatriate those inmates no longer deemed dangerous.

"I don't want these individuals to die. Obviously, the Pentagon is trying to manage the situation as best as they can. But I think all of us should reflect on why exactly are we doing this."

The president also said he was pleased with the cooperation from Moscow following the Boston bomb attacks, and said he and President Vladimir Putin had pledged to work together in a telephone call on Monday.

But he conceded that "old habits die hard" between security services in both nations which spent decades waging the Cold War, and that more could be done.

Obama also defended the FBI which, at the request of Russia, interviewed the now deceased Boston suspect Tamerlan Tsarnaev in 2011, but subsequently did not understand he later became radicalized.

"Based on what I've seen so far, the FBI performed its duties, the Department of Homeland Security did what it is was supposed to be doing.

"But this is hard stuff."