Mr. Obama told a veterans convention in Reno, Nevada Monday that the world is watching Syria. He said President Bashar al-Assad would be held accountable if his nation used poison gas and other deadly chemicals.

A Syrian Foreign Ministry spokesman said the government would never use chemical weapons against its own people, but would unleash them against what he called foreign invaders. He said the military is securely guarding the nation’s weapons stockpile.

Israeli Prime Minister Benjamin Netanyahu said Sunday he is deeply concerned about what would happen to Syria’s stockpile if the Assad government collapses. He said the Israeli army is preparing to act if Syrian weapons fall into the hands of Hezbollah or other Islamic militants.

Fighting between between rebels and government troops continued Monday in the capital, Damascus, and the northern city of Aleppo as the European Union tightened sanctions against Syria.

EU members strengthened their arms embargo, blacklisted nearly 30 people and companies associated with the Assad government, and banned the Syrian national airline from landing in EU countries.