UNITED NATIONS—Russia on Saturday vetoed a U.N. Security Council resolution calling on Syrian President Bashar Assad to step aside hours after a reported government massacre in Homs emboldened Western powers to push for a vote despite Moscow's objections.

The U.N.'s failure to pass the resolution means Arab States must now go back to the drawing board and figure out a new strategy for trying to solve the conflict. Analysts believe Gulf states eager to see President Assad unseated may quietly step up financial and arms support...