DAMASCUS—Arab leaders said Tuesday that Syria had used chemical weapons against its population, while Western governments discussed military responses to the attacks, bolstering the case for U.S. military strikes against the regime of Syrian President Bashar al-Assad.

The Arab League demanded an international response to what it called the "heinous crime" of last week's alleged chemical attack near Damascus that activists and rebels said left more than 1,000 Syrians dead. But the U.S.'s major Arab allies have stopped...