AFP/ California

Two heavily-armed suspects, a man and a woman in their 20s, have died in a shootout with police after killing 14 people at a Christmas party in California in America's deadliest mass shooting in three years.

Police identified the two as Syed Farook, a 28-year-old US citizen who worked for the local county, and Tashfeen Malik, 27, whose nationality was unknown. They said the suspects were either married or in a relationship.

Jarrod Burguan, San Bernardino's police chief, confirmed that both were dead, and that police no longer believed a third suspect was at large.

The shooters targeted a year-end party taking place at a social services building in San Bernardino, about an hour's drive east of Los Angeles, killing 14 people and wounding 17 more.

"We don't have the motive at this point," the police chief said. "We have not ruled out terrorism."

The massacre drew an angry response from President Barack Obama, who once again urged Congress to pass tougher gun control measures to stem the spiral of violence.

According to Burguan, Farook was a county employee and environmental specialist within the public health department.

He and Malik were dressed in military-style gear and carried assault weapons as they burst into the auditorium where the bloodbath took place, lent out for the holiday party by the Inland Regional Center for the disabled.

Burguan said Farook had attended the Christmas party organized by the health department and left after an apparent dispute, only to return a short time later and carry out the carnage.

"He did leave the party early under some circumstances that were described as angry or something of that nature," he said.

- 'Twisted mindset' -

But Burguan said this was unlikely to be a crime committed in the heat of the moment.

"Based upon what we have seen and based upon how they were equipped, there had to be some degree of planning that went into this," he said.

"So I don't think that he just ran home, put on these types of tactical clothes, grabbed guns and came back on a spur of the moment thing."

FBI agents in the early evening raided an apartment in the nearby town of Redlands, where the two suspects were seen before the police chase that ended in a shootout a few miles from the Inland Center.

The California chapter of the Council on American-Islamic Relations (CAIR) denounced the killings as Farook's brother-in-law, Farhan Khan, said he had no clue what prompted the carnage.

"I am in shock that something like this could happen," a visibly shaken Khan told a press conference organized by CAIR.

The Los Angeles Times quoted some of Farook's co-workers as saying he had previously travelled to Saudi Arabia and returned with a new wife. The couple had a baby and appeared to be "living the American dream," Patrick Baccari, a fellow inspector who shared a cubicle with Farook, told the paper.

Hussam Ayloush, CAIR's executive director in Los Angeles, said: "The Muslim community stands shoulder to shoulder with our fellow Americans in repudiating any twisted mindset that would claim to justify such sickening acts of violence."

- Deadliest since Sandy Hook -

It was the country's deadliest shooting since December 14, 2012, when a young man killed 26 people, including 20 children, at Sandy Hook elementary school in Newtown, Connecticut.

Burguan earlier told reporters the attackers "came prepared to do what they did, as if they were on a mission."

Investigators were looking at the possibility they had left behind an explosive device, he added.

Witnesses at the scene recounted barricading themselves in offices and hiding as the sound of gunfire erupted.

Mark Stutte said his daughter was attending the party organized by the county's public health department and called him terrified while hiding in a restroom as gunshots rang out in the background.

"It was really, really super scary," he told local TV, as he wept. "I'm far away. I couldn't do anything for her."

Lavinia Johnson, the center's executive director, said staff hid inside the building that employs several hundred people and waited to be evacuated.

"It was a very traumatic event for us," she told CNN.

- 'Pattern of mass shootings' -

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

"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," he 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."

Wednesday's shooting, which came less than a week after a gunman killed three people at a family planning center in Colorado, was certain to further stoke the bitter debate about gun control in the United States.

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

According to the site Mass Shooting Tracker, the latest attack brings to 352 the number of mass shootings in the United States so far this year. A mass shooting is defined as four or more people shot in one incident.