The FBI says Wednesday's gun massacre in San Bernardino is being investigated as an "act of terrorism" as investigators have uncovered evidence of "extensive planning".

David Bowdich, assistant director of the FBI's Los Angeles office, said the couple behind the "horrific" attack tried to destroy evidence, including crushing two mobile phones.

The husband and wife dumped the devices in a bin near the crime scene, but officials found the handsets and are hoping to uncover the suspects' digital footprint.

Mr Bowdich said the attackers - Syed Farook and Tashfeen Malik - had "telephonic connections" with the subjects of FBI investigations, but neither of them was on the intelligence radar.

FBI Director James Comey said on Friday there is no current indication the couple were part of a larger organised network, group or cell.

Aamaq, a news agency affiliated with the Islamic State (IS) group, says the San Bernardino shooters were "supporters" of the extremist group, though it stopped short of claiming responsibility for the attack.

It also emerged that Tashfeen Malik, the woman who was part of a couple who killed 14 in a massacre in California on Wednesday, had pledged her allegiance to the leader of the Islamic State.

Sky News reports that Investigators have told US media that the 27-year-old had peldged her allegiance to leader of ISIS Abu Bakr al-Baghdadi on a Facebook page held under false name. Investigators say she deleted the page before the attacks.

Along with spouse Syed Farook, 28, the Pakistan born Malik killed 14 people in an attack on a social services centre in San Bernardino on Wednesday. The pair were killed in an ensuing gun battle with police that followed a car chase.

Malik had been in the US on a K1 fiancee visa, and was sponsored by the American born Farook.

Farook had been in touch with Islamic extremists online, according to US intelligence services.

Reuters reports that the family claim to have been contacted by Pakistani intelligence services.

Farook's brother-in-law has told US media he was "a bad man" but not a radical.

While the motive for the attack remains unclear what is known so far is that Farook stormed out of his Christmas work party at the social services centre on Wednesday before returning with Malik, the pair armed with automatic rifles and wearing combat-style gear, to begin the attack.

Police say they found a dozen pipe bombs and close to 5,000 rounds of ammunition in a search of the couple's property.