U.S. Secretary of State John Kerry said on Friday it might be possible for the Syrian government and rebel forces to cooperate against Islamic State militants without Syrian President Bashar al-Assad having first left power. Kerry was asked at a news conference during a visit to Greece whether Assad's departure was a precondition for Western-backed rebels to cooperate with government troops against IS, which has captured a swathe of Syria and Iraq and carried out a string of attacks in other countries.