President Barack Obama says the US "will not be terrorised" by Wednesday's mass shootings in San Bernardino, California.

"We are strong. And we are resilient," he said in his weekly radio address, adding that it was "entirely possible" the two attackers had been radicalised.

Syed Rizwan Farook, 28, and his wife Tashfeen Malik, 27, killed 14 people before being shot dead by police.

The FBI is investigating the shootings as an "act of terrorism".

The Islamic State militant group on Saturday hailed the attack, saying it had been carried out by "two supporters of the Islamic State".

The statement, carried by IS's al-Bayan radio, gave no indication that the group had any involvement in planning the shootings at the Inland Regional Center social services agency.

"We know that Isil [IS] and other terrorist groups are actively encouraging people - around the world and in our country - to commit terrible acts of violence, oftentimes as lone wolf actors,"  Obama said in his address.

"All of us - government, law enforcement, communities, faith leaders - need to work together to prevent people from falling victim to these hateful ideologies."

Farook, who worked for the local health department, and Malik opened fire on an office Christmas party, killing 14 people and wounding 21.

FBI officers are trying to recover data from two mobile phones that were found crushed in a waste bin near the scene.

On Friday lawyers representing the attackers' family said the relatives were "in complete shock" and that they had no idea the two had been capable of such an attack.

Syed Rizwan Farook's sister, Saira Khan, told CBS News: "I can never imagine my brother or my sister-in-law doing something like this, especially because they were happily married, they had a beautiful six-month-old daughter."

Tashfeen Malik was born in Pakistan and lived for 20 years in Saudi Arabia before moving back to her native country to go to university.

She and Farook, a US national, met on Muslim dating websites, the New York Times quotes officials as saying. The new couple spent about a week in Saudi Arabia last year, before returning to the US together.

She was granted a visa allowing people to enter the US to marry American citizens.  Chesley said Malik was very conservative. She did not interact with male family members and wore a burka, he said.

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It is the deadliest mass shooting in the US since 26 people were killed at a school in Connecticut in 2012.

The couple used handguns and automatic weapons that had been legally purchased in the US, police say.

Bomb equipment, weapons and thousands of rounds of ammunition were later found in their home.

In response to the shooting, the New York Times ran an editorial calling for stricter gun controls on the front page of Saturday's print paper. It is the first time since 1920 that the paper has run an editorial on page one.

"It is a moral outrage and national disgrace that civilians can legally purchase weapons designed to kill people with brutal speed and efficiency," the opinion piece said.

"America's elected leaders offer prayers for gun victims and then, callously and without fear of consequence, reject the most basic restrictions on weapons of mass killing," it added.

At the scene: David Willis, BBC News, California

The bomb-making equipment and the thousands of rounds of ammunition have all been removed, and the tan-coloured townhouse which Syed Farook and Tashfeen Malik called home has now returned to normal.

On Friday, the FBI completed their search of the property on this leafy suburban street, and after confiscating notebooks and computers - and even Christmas tree lights - handed the property back to its owner.

Waiting reporters were allowed a peek behind the venetian blinds, hoping to get some degree of insight into the life of the "clean-cut young man" and his young bride and baby, who never caused problems and always paid their rent on time.

And as we piled across the threshold, we encountered not the remnants of some medieval torture chamber - or even the evidence of a fanatical terrorist cell - but all the trappings of domestic mundanity: powdered baby food in the kitchen, a cot in the upstairs bedroom, nappies, books and tapestries and several copies of the Quran.

FBI Director James Comey said earlier that the investigation was in its early stages and that the couple may have been "potentially inspired" by foreign terror groups.

The FBI is also investigating reports that Malik had posted a message on Facebook pledging allegiance to IS.