(MCT) — WASHINGTON — President Barack Obama warned the embattled leader of war-torn Syria on Monday not to use chemical weapons against rebels fighting to topple his regime, as the United States voiced rising concern that he may be preparing to do so and consulted with regional allies on a range of responses.

Obama’s admonition to Syrian President Bashar Assad was his most strident since he drew a “red line” in August that the movement of chemical or biological weapons out of their secure sites, or their use, would bring U.S. military intervention in the country’s vicious civil war.