Police officers conduct a manhunt after a shooting rampage in San Bernardino, California December 2, 2015. [Photo/Agencies]

BEIRUT - Islamic State said in an online radio broadcast on Saturday that two followers of the Islamist militant 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 attacked several days ago a centre in San Bernadino in California," the group's daily broadcast al-Bayan said.

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

Gunmen who died in a shootout with police opened fire in Wednesday's attack, killing 14 people and wounding 17 others before fleeing, authorities said.

The two suspects were identified as Syed Rizwan Farook, 28, and Tashfeen Malik, 27, who San Bernardino Police Chief Jarrod Burguan said were in a relationship, possibly married or engaged. He said they were believed to be the only shooters involved in an attack that required some degree of planning.