As many as three gunmen opened fire inside a centre for the disabled in San Bernardino, California on Wednesday, killing at least 14 people and wounding 14 others before fleeing the scene, officials said.

San Bernardino: As many as three gunmen opened fire inside a centre for the disabled in San Bernardino, California on Wednesday, killing at least 14 people and wounding 20 others before fleeing the scene, officials said. A suspect is "down", police said.

"I know there is an active scene, there were shots fired and a suspect is down," Sergeant Vicki Cervantes of San Bernardino Police told reporters.

A body could be seen near a bullet-riddled SUV, with dozens of police vehicles swarming around, according to live television footage.

The deadly incident was the latest in a series of mass shootings in the United States, a spiral of violence that has exasperated President Barack Obama, who once again called on Congress to pass tougher gun control measures.

San Bernardino police chief Jarrod Burguan told reporters the shooters were armed with long guns and "came prepared to do what they did, as if they were on a mission."

He said that while the motive and identities of the attackers were not yet known, "at a minimum, we have a domestic terrorist type situation that occurred here."

"The preliminary number we have right now is that there are 14 people that are deceased. And our count is another 14 people that have been taken to various hospitals for significant injuries," Burguan said.

San Bernardino County Sheriff John McMahon said courts and local city and county buildings in the region had been placed on lockdown, and police presence had been boosted at area schools as the manhunt for the suspects continued.

Heavily armed SWAT teams, firefighters and ambulances swarmed the scene, as police warned residents away.

The shooting took place inside a packed auditorium at the Inland Regional Centre, a huge facility that employs several hundred people and assists people with developmental disabilities.

Brandon Hunt, who works at the centre, said a banquet for county personnel was being held at the auditorium.

Lavinia Johnson, the centre's executive director, told CNN that her staff were waiting to be evacuated from the site.

"Our staff are just patiently waiting right now. It was a very traumatic event for us. And we're just waiting to be evacuated to another location."

Officials said police were going through the building room by room and checking reports of a suspicious package that may have been left by the gunmen.

Family members of employees at the site rushed to the area on hearing of the shooting and were seen frantically trying to get information about their loves ones

"It was an hour until I learned my daughter was OK," said Olivia Navarro, 63. "She was frightened when I spoke to her but she seemed calm."

'Stop gun violence'

Obama, who just last week made a plea for action on gun control after three people were killed at a family planning centre in Colorado, again voiced anger.

"The one thing we do know is that we have a pattern now of mass shootings in this country that has no parallel anywhere else in the world," Obama told CBS News.

"There are some steps we could take, not to eliminate every one of these mass shootings, but to improve the odds that they don't happen as frequently."

The Democratic presidential hopeful Hillary Clinton tweeted: "I refuse to accept this as normal. We must take action to stop gun violence now."

The city fire department said on Twitter it responded to initial reports of a "20 victim shooting incident."

Television footage showed dozens of people coming out of the building with their hands raised and walking to a parking lot, ringed by heavily armed police.

A spokeswoman for Loma Linda University Medical Centre said it was expecting an unknown number of patients from the shooting, according to CNN.

Inland Regional Centre serves thousands of people with developmental disabilities in San Bernardino and Riverside counties, according to its Facebook page.

The centre provides services to more than 30,200 people.

Wednesday's shooting was certain to further stoke the bitter debate about gun control in the United States.

In Oregon in October, a gunman killed nine people at a community college before turning the gun on himself.

According to the site Mass Shooting Tracker, there have been 351 mass shootings in the United States so far this year. A mass shooting is defined as four or more people shot in one incident.

Here's a look at some of the deadliest shootings since 2012:

2 December 2015: As many as three gunmen believed to be wearing military-style gear opened fire at a social services centre in San Bernardino, California, killing at least 14 people and wounding more than a dozen, authorities said. No one has been arrested in the shooting, and a motive is unknown.

1 October 2015: A shooting at Umpqua Community College in Roseburg, Oregon, left 10 people dead and seven wounded, authorities said. Shooter Christopher Harper-Mercer, 26, exchanged gunfire with police then killed himself.

17 June 2015: Dylan Roof, 21, shot and killed nine African-American church members during a Bible study group inside the Emanuel African Methodist Episcopal Church in Charleston, South Carolina. Police contend the attack was racially motivated. Roof faces nine counts of murder in state court and dozens of federal charges, including hate crimes.

23 May 2014: A community college student, Elliot Rodger, 22, killed six people and wounded 13 others in shooting and stabbing attacks in the area near the University of California, Santa Barbara, campus. Authorities said he apparently shot himself to death after a shootout with deputies.

16 September 2013: Aaron Alexis, a mentally disturbed civilian contractor, shot 12 people to death at the Washington Navy Yard before he was killed in a police shootout.

—July 26, 2013: Pedro Vargas, 42, went on a shooting rampage at his Hialeah, Florida, apartment building, killing six people before being shot to death by police.

14 December 2012: In Newtown, Connecticut, an armed 20-year-old man entered Sandy Hook Elementary School and used a semiautomatic rifle to slay 26 people, including 20 first graders and six adult school staff members, before killing himself.

27 September 2012: In Minnesota's deadliest workplace killing spree, Andrew Engeldinger, who had just been fired, pulled a gun and fatally shot six people, including the company's founder. He also wounded two others at Accent Signage Systems in Minneapolis before killing himself.

5 August 2012: In Oak Creek, Wisconsin, a gunman killed six worshippers at a Sikh Temple before taking his own life.

20 July 2012: James Holmes, 27, killed 12 people and wounded 70 in Aurora, Colorado, movie theater.

2 April 2012: Seven people were killed and three were wounded when a 43-year-old former student opened fire at Oikos University, in Oakland, California. One Goh was charged with seven counts of murder and three counts of attempted murder but psychiatric evaluations concluded he suffers from long-term paranoid schizophrenia and is unfit to stand trial.

With inputs from agencies