Islamic State's al-Bayan radio said Saturday that a shooting spree in California that killed 14 people was carried out by two of its supporters, a day after US authorities said they were investigating the crime as an act of terrorism.

The station, however, did not claim the attackers as members of the militant group.

US President Barack Obama's national security team told him in a briefing on the investigation Saturday that the perpetrators had been radicalized, but that there was no evidence they were part of an organized terrorist group.

Obama was briefed on the investigation by FBI Director James Comey, Attorney General Loretta Lynch, Secretary of Homeland Security Jeh Johnson and the intelligence community, the White House said.

They "highlighted several pieces of information that point to the perpetrators being radicalized to violence to commit these heinous attacks," spokesman Josh Earnest said.

Obama ordered the team to "take all measures necessary to continue to protect the American people."

The accused perpetrators of Wednesday's attack in San Bernardino are a husband and wife, and the US Federal Bureau of Investigation (FBI) said the woman, Tashfeen Malik, 29, had pledged allegiance to Islamic State in a Facebook post shortly before or during the attack.

Obama vowed Saturday that the United States "will not be terrorized" while acknowledging the threat of militant groups like Islamic State encouraging people to commit terrorist acts.

"It is entirely possible that these two attackers were radicalized to commit this act of terror, and if so, it would underscore a threat we've been focused on for years: the danger of people succumbing to violent extremist ideologies," Obama said in his weekly address.

Using an acronym for Islamic State, he said, "We know that ISIL and other terrorist groups are actively encouraging people around the world and in our country to commit terrible acts of violence, often times as lone wolf actors."

Comey said Friday that there was no evidence that Malik and her husband, Syed Rizwan Farook, 28, were part of an organized terrorist group or cell.

The attack would fit with Islamic State efforts to use social media and the internet to inspire so-called lone wolf terrorists to commit attacks abroad, said Javier Lesaca, a terrorism expert at George Washington University in Washington.

Farook's colleagues at the county health department were sprayed with gunfire at a conference centre in the city east of Los Angeles. The couple were killed hours later in a shootout with police.

Malik was Pakistani and moved to the United States from Saudi Arabia in 2014 to marry Farook. Pakistani intelligence officials and relatives said she comes from a "highly radicalized" Sunni family, and her step-uncle Javed Rabbani Malik said her immediate family grew more radicalized after her father moved them to Saudi Arabia when Malik was a child.

Pakistani intelligence sources also said Malik had had contact in the past with radical Islamic cleric Maulana Abdul Aziz at Islamabad's Red Mosque, which was stormed by the military in 2007 for having links with the al-Qaeda terrorist network. A mosque spokesman said he could not confirm whether they had met.

Obama urged leaders in US government, law enforcement, communities and religious leaders "to work together to prevent people from falling victim to these hateful ideologies."

Obama also spoke Saturday with French President Francois Hollande about the attacks.

"The two leaders pledged continued cooperation between our two governments and with those of our allies and friends to fight terrorism, both abroad and at home," the White House said.

Obama and Hollande had also pledged close cooperation following the Paris terrorist attacks last month.

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