BEIRUT — A U.N. humanitarian aid convoy inside Syria was hit by airstrikes, U.N. officials said, as the Syrian military declared Monday that the week-long U.S.-Russian brokered cease-fire had failed. With the truce apparently teetering on the brink of collapse, the U.S. said it’s prepared to extend the agreement, and Russia – after blaming rebels for the violations – suggested it could still be salvaged. U.N.