U.S. President Barack Obama is warning that if Syrian President Bashar al-Assad's government were to use chemical weapons in its fight against rebel forces, it could significantly affect how the United States responds to the conflict.

Mr. Obama told reporters at the White House Monday that any Syrian move to use those weapons would be a “red line” that would change the scope of the situation.

The president noted that he has not at this point ordered any U.S. military engagement in Syria, but he added that the United States is monitoring the situation very carefully.

Earlier Monday, members of a United Nations observer mission left the Syrian capital, Damascus, after the expiration of their mandate to watch over a cease-fire that never took hold.

The monitors who departed for Lebanon Monday are some of the last remaining from a group that at one time included 300 observers.

The rest of the team is expected to be out of Syria by the end of the week. The mission's mandate ended Sunday, and the U.N. Security Council agreed to leave only a small liaison office in Syria to support any future peace efforts.

Observers were sent to Syria as part of former international envoy Kofi Annan's plan to halt the ongoing violence in the country. But the bloodshed continued, leading the mission to suspend its work in June.

Annan quit earlier this month, complaining that he could not fulfill his mission because of international divisions on Syria and escalating attacks by Syrian government and rebel forces.

Former Algerian diplomat Lakhdar Brahimi has replaced Mr. Annan as the new U.N. special envoy for Syria.