WASHINGTON (REUTERS, AFP) - United States President Barack Obama on Tuesday said there was evidence that chemical weapons were used inside Syria, but questions remained about their use.

Again calling it a "game changer", Mr Obama told reporters that what was still not known is how the chemical weapons were used, when they were used and who used them.

He said the US would "rethink" its policy on Syria if he receives proof that the Damascus regime has used chemical weapons.

However, he expressed caution about intelligence reports, telling a White House news conference: "I've got to make sure I've got the facts. That's what the American people would expect."

Syrian President Bashar al-Assad's government has been accused of using weapons loaded with nerve agents against targets in civilian areas during its battle with an armed rebellion against his rule.

Washington has previously warned that the use of so-called weapons of mass destruction would be a "red line" that Mr Assad must not cross if he is to avoid triggering an international response.

Mr Obama did not rule out eventual US military action, but remained cautious.

"By game changer, I mean we would have to rethink the range of options that are available to us. We are invested in trying to bring back a solution inside of Syria," he told reporters.

"Obviously, 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.

"As early as last year, I asked the Pentagon, our military, our intelligence officials to prepare for me what options might be available, and I won't go into the details of what those options might be," he continued.

"But clearly that would be an escalation in our view of the threat to the security of the international community, our allies and the United States and that means that there are some options that we might not otherwise exercise that we would strongly consider."