"This was an act of terrorism, designed to kill innocent people," Xinhua quoted Obama as saying on Sunday.
It was clear that the two shooters had gone down the dark path of radicalisation, embracing a perverted interpretation of Islam that calls for war against the US and the West, Obama said.
He called on Americans not to turn against each other, saying the Muslim Americans were "friends, coworkers" of the US.
Obama also said the country should carry out stricter screening for those who come to the US without visa.
The Congress should act to make sure no one on a "no fly list" was able to buy a gun, Obama said in his third Oval Office address during his seven-year presidency.
The US will continue to provide training and equipment to thousands of Iraqis, he said.
He also reiterated objection to sending US ground troops abroad to fight against the Islamic State (IS).
Two gunmen, US citizen Syed Farook, 28, and his wife Tashfeen Malik, 27, shot down 14 people and injured 21 others on Wednesday at a social services centre.
According to local police, "some degree of planning" was involved in the shooting spree.
Obama's address came less than a week after southern California shooting spree, and less than a month after the deadly attacks in Paris, for which the IS had claimed responsibility.
The November 13 attacks in Paris left at least 130 people killed and over 350 others injured.