A 60-kilometer (37 mile) stretch of the Interstate 5, one of California's biggest transport routes, was closed in a mountainous region between Castaic, northeast of Los Angeles, and Wheeler Ridge, the California Department of Transportation (Caltrans) said in a statement on its website.

"Motorists are advised to use an alternate route," the statement said.

California is shivering under a cold front which has affected the western United States since last week, sending temperatures to near-record lows.

Governor Arnold Schwarzenegger has declared a state of emergency because of the weather which is threatening to devastate the traditionally sun-baked state's citrus fruit industry.

Schwarzenegger warned Tuesday that losses to the citrus fruit sector could hit the one billion-dollar mark.