The interstate is the main highway from the region to Las Vegas.
More than 200 firefighters aided by 10 aircraft are battling the flames in hot, dry weather on brush-dotted foothills of the San Bernardino Mountains.
An area known as Mathews Ranch has been ordered evacuated. It's not clear how many people that involves.
Television news reports show the line of flames has jumped the freeway and sent an enormous mushroom shaped cloud into the sky.