US President Barack Obama called the recent shooting rampage in southern California "an act of terrorism" and warned that terrorist threat has evolved into a new phase.

"This was an act of terrorism, designed to kill innocent people," said Obama 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 radicalization, embracing a perverted interpretation of Islam that calls for war against America and the West," Obama said.

Widely seen as the most sobering communication channel a US president has, the Oval Office address came this time amid widespread jitters in the country over further terrorist attacks inspired by the Islamic State (IS) extremist group, also known as ISIL or ISIS.

In his prime-time speech on Sunday night, Obama told millions of Americans that their fear of such attacks as what happened Wednesday at the southern California social services center 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 911, 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 on Sunday there was a serious threat posed by extremist groups' online calling for lone-wolf type of attacks against the United States and other Western countries.

Just hours before Obama's Oval Office address, US Attorney General Loretta Lynch told reporters that the "inspired-terrorist model" is becoming both 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 in an interview with the US TV network NBC News.

Two gunmen shot down 14 people on Wednesday at a southern California social services center and injured several others. According to local police, "some degree of planning" was involved in the shooting spree.

The suspects, 28-year-old US citizen Syed Farook and his 27-year-old wife Tashfeen Malik, had 1,600 rifle and 9-millimeter rounds of ammunition when they were killed, and 2,000 9-millimeter rounds and 2,500 .223 rifle rounds at home, as well as 12 pipe bombs and tools to make bombs, according to the police.

Those did not include the hundreds of rounds they fired in the shooting and gunfire with the police.

According to local police, motives of the shooting spree remained unknown. However, citing investigators familiar with the case, local media said at least one of the shooters had pledged allegiance to the IS.

On Sunday, Obama again 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 a terrorist organization overseas or that they were part of a broader conspiracy here at home," said Obama.

The Oval Office address also came as Obama was facing mounting pressure from Republicans to change his current counter-IS strategy and send more ground troops to Syria, a demand again dismissed by Obama on Sunday.

"We should not be drawn once more into a long and costly ground war in Iraq or Syria. That's what groups like ISIL want," said Obama.

"They know they can't defeat us on the battlefield... They also know that if we occupy foreign lands, they can maintain insurgencies for years, killing thousands of our troops and draining our resources, and using our presence to draw new recruits," said Obama.

Since the outset of the US-led coalition campaign against the IS, now in control of wide swaths of territories both in Syria and Iraq, the Obama administration has been sticking to its original strategy of training local forces to conduct ground assaults against the group, together with launching air raids.

"That is how we'll achieve a more sustainable victory, and it won't require us sending a new generation of Americans overseas to fight and die for another decade on foreign soil," said Obama.

Despite his opposition to sending ground troops overseas, Obama announced in October his plan to send fewer than 50 US special operations troops to assist rebels in Syria.

The Pentagon early this month expanded the Obama administration's overseas deployment plan to include Iraq, saying that a new US specialized operations force of about 100 was being deployed to Iraq.

In its defence, the White House had argued that sending special operations forces, whose main mission was to train and support local forces, was different from large-scale, ground combat operations.

Despite the White House's efforts to downplay the significance of sending US special forces both to Syria and Iraq, the expansion of US military campaign to include more troops on the grounds was a major shift in the US counter-IS strategy, which has so far gone through a series of embarrassing moments.

The Pentagon's 500-million-dollar train-and-equip program got off to a disastrous start in Syria in July when members of the first round of 54 Syrian trainees were either killed or taken hostage by the Syrian affiliate of al-Qaida group, the Nusra Front, even before their fight against the IS started. Other remaining fighters reportedly fled.

Grilled in a hearing by US lawmakers in September, US General Lloyd Austin, Commander of US Central Command (CENTCOM), revealed that only "four or five" US-trained Syrian rebels had been positioned in Syria to fight the IS.

The Pentagon later corrected the number of US-trained Syrian rebels fighting the IS in Syria to nine.

The program was halted by the Pentagon in October.

Also in September, it was exposed that a cohort of US intelligence analysts complained that their reports on the campaign were altered by senior officials to present a rosier picture.

The complaint claimed that senior officials from the CENTCOM, including the director of intelligence and his deputy, had whitewashed the situations on the ground to fit the Obama administration's public narrative that the US was winning the battle against the IS.

Despite Obama's repeated remarks that the US military campaign had so far "contained" the IS, Joseph Dunford, chairman of the US Joint Chiefs of Staff, said early this month in a congressional hearing that "We have not contained ISIS."