Daesh said in an online radio broadcast on Saturday that two followers of the group had carried out Wednesday's attack on a social services agency party in California in which 14 people were killed.

"Two followers of Islamic State [Daesh] attacked several days ago a centre in San Bernadino in California," the group's daily broadcast al-Bayan said.

A news agency which supports Daesh said on Friday the attackers were followers of the group. U.S. government sources have said there is no evidence the attack was directed by the group, or that the organisation even knew who the attackers were.

A heavily armed man and woman in their 20s died on Wednesday in a shootout with police after killing 14 people at a Christmas party in California in America's worst mass shooting in three years.

Police identified the pair as Syed Farook, a 28-year-old U.S. citizen who worked for the Local County, and Tashfeen Malik, 27, whose nationality was unknown. They said the suspects were either married or engaged.

The shooters targeted a year-end party that was taking place at a social services center in San Bernardino, about an hour 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 America's epidemic of gun violence.

The killings marked the deadliest mass shooting in the U.S. since the Sandy Hook Elementary School massacre in Connecticut in 2012, in which 26 people were killed, including 20 children.

There have been at least 351 mass shootings in the U.S. since January, according to ShootingTracker.com, a website that tracks news reports on shooting incidents.