WASHINGTON - President Obama Thursday ruled out unilateral U.S. military action in Syria even if there is proof that Syrian forces used lethal chemical weapons in the nation's civil war.

"This is … an international problem," Obama said at a White House news conference with Turkish Prime Minister Recep Tayyip Erdogan. "It's not going to be something that the United States does by itself. And I don't think anybody in the region would think that U.S. unilateral actions ... would bring about a better outcome."

Obama's warnings since August that Syrian President Bashar Assad would cross a "red line" if his forces used poison gas in the war were widely viewed as a threat of U.S. military intervention.

But in recent weeks, with growing evidence indicating use of sarin nerve gas, Obama has made it clear that he wants proof before ordering a response. He has previously indicated that he would prefer a collective response, but Thursday was the first time he categorically ruled out action by the United States alone.

Experts say it may be difficult to prove that chemical weapons were used by Syrian troops and not rebel forces, as Assad's government has claimed. Even if proof is found, it may be difficult to persuade an international coalition to agree to a military response, analysts say.

Erdogan's visit was aimed at appealing to Obama to sharply increase pressure on Assad to end the conflict, which over more than two years has sent about 400,000 refugees into Turkey and threatens to destabilize the region.