Senate panel considers 'red lines' for US intervention in Syria

President Obama himself has said Assad would be held “accountable” for the use of chemical weapons.

“The world is watching and they will be held accountable if they make the tragic mistake of using those weapons," Obama said at a speech last week before the Veterans of Foreign Wars convention.

And Russia, one of Assad's closest allies, said Tuesday that it expects Syria to "unfailingly honor its international obligations” under the global convention banning the use of chemical weapons.

A second potential trigger, Kerry said, is “some very significant massacre,” a “bloodletting not dissimilar to what prompted President Clinton to move in the Balkans.”

A previous massacre of more than 100 civilians in Houla in May, however, was called a “game-changer” that could force China and Russia to support sanctions against Syria — something that never materialized.