Gunfire erupted at a holiday party at a social services agency in San Bernardino, California, on Wednesday, leaving 14 people dead and wounding 14 others, sparking an intense manhunt for up to three suspects, authorities said.

The shooting rampage about 60 miles (100 km) east of Los Angeles marked the deadliest U.S. gun violence since the massacre at Sandy Hook Elementary School in Newtown, Connecticut, in December 2012, in which 27 people, including the gunman, were killed.

San Bernardino Police Chief Jarrod Burguan said in a televised news briefing three hours after Wednesday's shooting that the suspects fled the building and were believed to have made their getaway in a dark-coloured sport utility vehicle.

With as many as three gunmen at large, authorities ordered a security "lockdown" of all local schools, as well as city and county buildings, and area hospitals were placed on alert, Burguan said.

Burguan said he knew no possible motive behind the attack, which unfolded at about 11 a.m. at the Inland Regional Center in San Bernardino, about 60 miles (100 km) east of Los Angeles.

"We have no information at this point that this is terrorist-related, in the traditional sense that people may be thinking," he added. "Obviously, at minimum, we have a domestic-type terrorist-type situation that occurred here."

He said the suspects were armed with rifles.

The shooting unfolded on the campus of the Inland Regional Center, one of 21 facilities set up by the state and run under contract by non-profit organizations to serve people with developmental disabilities, said Nancy Lungren, spokeswoman for the California Department of Developmental Services.

Lavinia Johnson, executive director of the facility, told CNN the suspects opened fire inside a conference building in the complex where a holiday party was being held for county health department personnel.

The police chief said he, too, understood that the shooting occurred in the midst of a holiday luncheon for employees.

The conference building sits adjacent to the two larger three-story buildings that house most of the agency's offices at the complex, Johnson said. Asked whether that meant that the Inland Regional Center staff and clients were safe, she said she understood they were being evacuated.

Dr Maxwell Ohikhuare, health officer for the San Bernardino County Department of Environmental Health, confirmed that county employees were at the Regional Center for a holiday party for the Department of Environmental Health.