Cairo (dpa) - 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.

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.

President Barack 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."

FBI Director James 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 investigation so far has developed evidence of radicalism by the killers and the potential for inspiration from a foreign terrorist organization," Comey said.

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."