PHOENIX — A sudden dust storm today spawned a series of collisions involving as many as 20 vehicles on Interstate 10 south of Phoenix, leaving at least four people dead.

Arizona Department of Public Safety officials said other casualties were airlifted to Phoenix hospitals with severe burns and other traumatic injuries.

A second series of accidents was reported farther south along the main route between Tucson and Phoenix. High winds and blowing dust were also reported west of Phoenix, and the highway patrol was urging drivers headed to Phoenix from California to drive with extreme caution.

The midday accidents led authorities to close I-10 between Phoenix and Tucson. It was expected to remain blocked for at least several hours.

Both directions of Interstate 40 in northern Arizona were also closed west of Flagstaff because of high winds and snow.

Traffic backed up for miles as I-10, a major east-west thoroughfare, was shut down in both directions near Casa Grande. Dust and thick gray smoke from burning vehicles billowed across the flat sprawl of farms and desert.

The highway patrol said a commercial vehicle exploded, so a hazardous materials team was dispatched to the scene.