SAN FRANCISCO (AFP) - A strong 6.0-magnitude earthquake jolted California early Sunday morning, the US Geological Survey said, though there were no immediate reports of deaths or serious damage.

The quake struck at 3:20 am (6.20 pm Singapore time) north-west of American Canyon, California, USGS said, at a depth of just 10.8km below the surface.

The power was out in Napa, about 9.7km from the earthquake's epicentre, as well as in other towns and cities nearby, affecting more than 10,000 households, according to the Pacific Gas and Electric Company.

The California Highway Patrol in the San Francisco Bay Area tweeted that it was "checking over crossings and bridges for obvious signs of structural integrity," and asked residents to report any signs of problems.

Residents of the cities of San Francisco, some 64.4km south-west, and Davis, just over 64.4km north-east, quickly took to Twitter reporting also feeling the quake.

Twitter user Tyson Winter wrote: "Shook violently here in Napa. Power's out."

Ms Ann Marie Christy,wrote that her mother, also in Napa, said the shaking "was very violent with a lot of broken glass." And in San Francisco, Mr Om Malik posted "Damn, woke me up. Was super long. # quake # sf." Further north, Mr Tim Kerbavaz tweeted: "Felt it in Davis too. Quite long."