Los Angeles, Dec 3 (IANS): One or more gunmen opened fire at a social services centre in the Southern California city of San Bernardino in the US on Wednesday, shooting as many as 20 people, authorities said.

The San Bernardino Sheriff's Department said in a Twitter post that the police has "confirmed one to three possible suspects" and multiple victims in the "active shooter" incident, Xinhua reported.

The San Bernardino City Fire Department said via its Twitter account that it was responding to reports of 20 victims.

Sgt. Vicki Cervantes, a spokeswoman for the San Bernardino Police Department, told media that up to three shooters were reported inside the Inland Regional Center, a non-profit offering services to individuals with developmental disabilities.

The suspects were heavily armed and were possibly wearing body armour, the police said.

The shooters were still active as of 12:00 p.m. local time (2000 GMT). Police and SWAT teams were searching the building to clear it and looking for more survivors.

US President Barack Obama was briefed on the shooting, a White House official told reporters.