US citizen Syed Farook, 28, and his wife Tashfeen Malik, 27, were involved in the attack in Californian city San Bernardino, the terror outfit said in its daily broadcast al-Bayan, Xinhua news agency reported.
According to police, "some degree of planning" was involved in the shooting spree.
Earlier, US' Federal Bureau of Investigation (FBI) chief James Comey said on Friday that the shooting was being investigated as potential terrorism, but there was "no indication" suggesting a terror angle.
"The investigation so far has delivered indications of radicalization by the killers and of potential inspiration by foreign terrorist organisations," Xinhua quoted Comey as saying.
However, Comey said so far there was no indication that pointed to the husband-and-wife shooters' affiliation to any group.
"We have no indication that these killers are part of an organised larger group or form part of a cell. There is no indication that they are part of a network," said Comey.
The couple shot down 14 people Wednesday at a Southern California social services centre and injured several. According to local police, "some degree of planning" was involved in the shooting spree.
It was revealed on Friday that the wife had pledged allegiance to the Islamic State (IS) on social media.