President Barack Obama called the apparent gassing of hundreds of Syrian civilians a "big event of grave concern" but stressed on Friday he was in no rush to embroil Americans in a costly new war.

As opponents of Syrian President Bashar al-Assad braved the frontlines around Damascus to smuggle out tissue samples from victims of Wednesday's mass poisoning, Obama brushed over an interviewer's reminder that he once called chemical weapons a "red line" that could trigger U.S. action.