President Barack Obama and visiting British Prime Minister David Cameron expressed limited optimism Monday that an upcoming international conference on Syria will lead to a political solution to its civil war.

"I'm not promising that it's going to be successful," Obama said at a joint news conference. "Frankly, sometimes once . . . the Furies have been unleashed in a situation like we're seeing in Syria, it's very hard to put things back together."

Cameron spoke of "an urgent window of opportunity before the worst fears are realized."