US president Barack Obama has said that the motives behind the San Bernardino married gunmen who killed 14 people at a centre for disabled people are possibly terror-related, though they remain unknown.

Speaking at a meeting with his national security team in the Oval Office of the White House in Washington, Obama pointed out that the FBI is now investigating the shootings in co-operation with local law enforcement authorities "to get to the bottom of this". He said it was possible the massacre was terror-related "but we don't know, possibly also workplace-related".

"There may be mixed motives involved in this, which makes the investigation more complicated," he said, adding that until there's an understanding of the workplace relationship between the individual and his superiors, all options are on the table.

Obama stressed that "we need to take basic steps to make it harder, not impossible" for people to carry out mass shootings. He added that Americans should not "feel as if there's nothing we can do.

"We can't just leave it to our professionals to deal with the problems of these kinds of horrible killings," he said. "I do think that as the investigation goes forward it's going to be important for all of us, including our legislatures, to see what we can do. ... So that when individuals decide they want to do somebody hard to make it a little harder for them to do it. Because right now it's just too easy."

Shooting took place at a Christmas party

The suspected gunmen Syed Farook, 28, and his wife Tashfeen Malik, 27 were later shot dead by authorities after a massive search in San Bernardino.

The shooting targeted a Christmas party for staff of the San Bernardino County's public health department, at the Inland Regional Centre training and support for people with disabilities. Farook was in attendance but left early after what was thought to be a dispute. "[He] did leave the party early under circumstances described as 'angry'," San Bernardino police Chief Jarrod Burguan told a news conference.

Farook returned later with his wife and the pair allegedly opened fire on the gathering. They were said to be wearing tactical-style gear, and at least 17 people were wounded in the assault. Police said they destroyed three suspected explosives devices similar to pipe bombs that were left at the scene.

The White House released an order to fly the American flag at half-staff at all public buildings, grounds, military bases and embassies, in honour of the victims of the shooting.