Washington: US President Barack Obama called the shooting rampage in the Californian city of San Bernardino "an act of terrorism" and warned that the terror threat has evolved into a new phase.
"This was an act of terrorism, designed to kill innocent people," Xinhua quoted Obama as saying on Sunday in his third Oval Office address during his seven-year presidency.
"It is clear that the two of them had gone down the dark path of radicalisation, embracing a perverted interpretation of Islam that calls for war against America and the West," Obama said.
Obama's address came amid widespread jitters in the country over further terrorist attacks inspired by the extremist group Islamic State (IS).
Obama told Americans that their fear of such attacks as what happened on Wednesday at California social services centre was justified.
"Over the last few years, however, the terrorist threat has evolved into a new phase," said Obama, seconds after he touted US counter-terrorism campaign in the wake of 9/11 attacks which, in his words, decimated Al-Qaeda's leadership.
"As we've become better at preventing complex multi-faceted attacks like 9/11, terrorists turn to less complicated acts of violence like the mass shootings that are all too common in our society."
Echoing US law enforcement and intelligence leaders, Obama said, in 2015 there was a serious threat posed by extremist groups' online calling for lone-wolf type of attacks against the US and other Western countries.
US Attorney General Loretta Lynch said the "inspired-terrorist model" was becoming more prevalent and in some ways harder to prevent.
"We have come from a time of the large-scale, planned, Al-Qaeda-style attacks to the encouragement of lone wolves -- Fort Hood, Chattanooga -- to the encouragement of people to act on their own," said Lynch.
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.
According to local police, motives of the shooting spree still remained unknown.
However, citing investigators familiar with the case, the media said at least one of the shooters had pledged allegiance to the IS.
On Sunday, Obama stressed that no evidence had so far pointed to the shooters' affiliation with any terrorist groups, including the IS.
"So far, we have no evidence that the killers were directed by terrorist organisation overseas or that they were part of a broader conspiracy here at home," said Obama.