SAN JOSE, Costa Rica (AFP) - President Barack Obama came close Friday to ruling out deploying U.S. troops to Syria, saying he did not foresee a scenario in which that would be beneficial to the United States or Syria.

"As a general rule, I don't rule things out as commander-in-chief because circumstances change and you want to make sure that I always have the full power of the United States at our disposal to meet American national security interests," Obama said.

"Having said that, I do not foresee a scenario in which boots on the ground in Syria -- American boots on the ground in Syria -- would not only be good for America but also would be good for Syria."

Speculation has mounted that the Obama administration could reverse its opposition to arming the rebels after the White House said last week that President Bashar al-Assad likely used chemical weapons on his people.

Obama has been reluctant to intervene in the war but faces mounting criticism that he has allowed the Assad regime to cross his own declared "red line" on using chemical weapons.

But the U.S. president has also stressed that more proof is needed for the United States to step up its involvement in a civil war that has already claimed more than 70,000 lives and is now in its third year.

Speaking during a visit to Costa Rica, Obama said there was evidence that chemical weapons had been used in Syria, but that "we don't know when, where or how they were used."

But he noted that any strong evidence of the Assad regime using such weapons would be a "game changer" because they could fall into the hands of groups like the militant group Hezbollah, based in neighboring Lebanon.

"In terms of any additional steps that we take, it is going to be based on, number one the facts on the ground, number two it's going to be based on what's in the interest of the American people and our national security," Obama said.

"As president of the United States I'm going to make those decisions based on the best evidence and after careful consultation because when we rush into things, when we leap before we look, then not only do we pay a price but often times, we see unintended consequences on the ground."

Experts say a military mission to secure the chemical weapons would require a large ground force and pose huge risks, with the outcome hinging on the quality of Western intelligence.

Former Pentagon chief Leon Panetta, who stepped down in February, had told lawmakers that he and the US military's top officer, General Martin Dempsey, had recommended arming the rebels but were overruled.

In Syria meanwhile, government troops bombarded Sunni areas of the Mediterranean city of Banias, a monitoring group said, warning of a new "massacre" that left at least 50 people dead.