Washington (AFP) - President Barack Obama will make a rare primetime address Sunday laying out plans to keep Americans safe and defeat the Islamic State group, days after 14 people were shot dead in California.

Obama's top law enforcement officer, US Attorney General Loretta Lynch, said the president hoped to reassure the US public, spooked by a seemingly new type of terror attack on the home front.

IS has praised the San Bernardino shooters, Syed Farook and his Pakistani wife Tashfeen Malik, as "soldiers" of its self-proclaimed caliphate, while stopping short of claiming outright credit for the attack.

Obama declared Saturday that the United States "will not be terrorized."

"I think what you'll hear the president say is to call on the American people to not give into fear," Lynch told NBC television. "You may hear him call on Congress to review measures and take action as well."

The Oval Office address is set for 8 pm Sunday (0100 GMT Monday), and will tackle "the broader threat of terrorism, including the nature of the threat, how it has evolved, and how we will defeat it," according to the White House.

The FBI is probing Wednesday's shooting at an office party in San Bernardino as an "act of terrorism." If confirmed to be terror-related, it would be the deadliest such assault on American soil since the September 11, 2001 attacks.

- 'Soldiers of the caliphate' -

Four days after the carnage, the motives of the shooters for donning tactical gear and opening fire on an office party full of Farook's co-workers remain unclear.

The seemingly quiet married couple -- who left their six-month-old baby with her grandmother as they went to commit mass murder -- died in a hail of police bullets hours later.

Top security officials believe the pair had been radicalized, with investigators probing reports the 29-year-old Malik had pledged allegiance to IS in a Facebook post.

But the White House and FBI both say there are no signs they were part of a larger terror group.

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Supporting that analysis, an English radio broadcast by IS praised them as "soldiers of the caliphate" and martyrs, but did not say they were members of the group.

Several relatives have voiced shock at the killing spree, and family attorneys have said the couple -- while devout Muslims -- were no radicals.

But in an interview with Italian newspaper La Stampa published Sunday, Farook's father offered a contrasting story, saying his son approved the ideas of the Islamic State group and was fixated on Israel.

"He said he agreed with (IS chief Abu Bakr) al-Baghdadi's ideas for creating the Islamic State, and he was obsessed by Israel," La Stampa quoted the father, also named Syed Farook, as saying.

- Self-radicalized -

Investigators suspect that Malik, who came to the United States on a fiancee's visa and spent extended periods of time in both Pakistan and Saudi Arabia, may have radicalized her husband.

The probe is trying to establish if she had contact with Islamic radicals in either country.

House Homeland Security chair Michael McCaul said Sunday the "wild card" was Farook's wife, Malik.

Despite its leading role in overseas military actions against the Islamic State group, the United States does not face a domestic jihadist threat on the scale of its European allies.

But the IS claim to have inspired the San Bernardino massacre spotlights the threat of homegrown, self-radicalized extremists.

"We have moved to an entirely new phase in the global terrorist threat and in our homeland security efforts," Homeland Security Secretary Jeh Johnson told The New York Times.

"This requires a whole new approach, in my view."

This year has seen record numbers of arrests of suspected extremists, many of them victims of FBI sting operations but others with ties to foreign groups deemed "terrorist" under US law.

According to a major study of US jihadists released this week by George Washington University, 250 Americans have traveled or attempted to travel to Syria or Iraq to fight for the so-called IS "caliphate."

There are 900 active investigations against alleged IS sympathizers across all 50 US states and 71 have been charged with "IS-related" activities since March.

- 'Gun epidemic' -

Obama, who last gave an Oval Office address in August 2010 to mark the end of US combat operations in Iraq, was also expected to address the deeply divisive question of gun control.

The California rampage, which also wounded 21, was the worst mass shooting in three years in a country where such killings have become routine.

The New York Times on Saturday published a front-page editorial -- the first since 1920 -- calling for an end to "the gun epidemic in America."

But a powerful conservative movement -- spearheaded by leading Republican presidential candidates -- is arguing precisely the opposite.

Donald Trump, the Republican frontrunner for the White House, on Sunday repeated his claim that in San Bernardino, as in Paris last month, the victims were "like sitting ducks" because they were unarmed.

"In Paris, they had no guns. In California they had no guns. Only the bad guys had the guns," he said.