Do more against IS

US President Barack Obama vowed Sunday that America would destroy the Islamic State group and hunt down its followers at home or abroad, in a rare address from the Oval Office to a jittery nation.

He urged Muslims in America and around the world to "decisively and unequivocally reject the hateful ideology that groups like ISIL and al-Qaeda promote."

Facing questions about his leadership and strategy, Obama harnessed the highest trappings of US power to calm a country rattled by a rampage in California that killed 14 people.

"After so much war, many Americans are asking whether we are confronted by a cancer that has no immediate cure," Obama said in a solemn speech, adding that the San Bernardino massacre was evidence of an "evolving" and increasingly home-grown threat.

As a father of two daughters, Obama said, he could imagine himself or his kin in San Bernardino or in Paris, where scores of people were killed last month in attacks claimed by Islamic State.

"Here's what I want you to know," he said. "The threat from terrorism is real, but we will overcome it. We will destroy ISIL and any other organization that tries to harm us," he said.

"Our military will continue to hunt down terrorist plotters in any country where it is necessary."

He detailed a multi-pronged strategy against the jihadists that will rely as much on community action, technology and countering propaganda as military force.

'DARK PATH OF RADICALIZATION'

It is just the third time Obama has delivered an Oval Office address -- used by presidents since Harry Truman to convey resolve in the face of a national crisis.

A senior administration official said the speech was designed to convey the seriousness with which Obama was taking the shootings, which are being investigated as a terror attack.

On Wednesday, US-born Syed Farook and his Pakistani wife Tashfeen Malik dropped off their six-month-old daughter with her grandmother, donned tactical gear and burst into an office party full of Farook's co-workers, spraying them with bullets.

Obama said the pair "had gone down the dark path of radicalization."

"They had stockpiled assault weapons, ammunition, and pipe bombs. So this was an act of terrorism."

But he added that there was "no evidence that the killers were directed by a terrorist organisation overseas or that they were part of a broader conspiracy here at home."

Both shooters died in a hail of police bullets, leaving questions about how, when and why they may have become radicalised.

The Islamic State has praised the attackers as "soldiers" of its self-proclaimed caliphate, while stopping short of claiming outright credit.

MUSLIM ALLIES

Woven throughout Obama's address was a plea for unity.

"We cannot turn against one another by letting this fight be defined as a war between America and Islam," he said, facing down some of his shrillest critics who have called for a registry of Muslim-Americans.

"ISIL does not speak for Islam. They are thugs and killers. Part of a cult of death" he said.

"If we're to succeed in defeating terrorism, we must enlist Muslim communities as some of our strongest allies rather than push them away through suspicion and hate."

After two decades battling jihadism, Americans appear increasingly divided on the nature of the problem and how to respond.

According to a new CNN/ORC poll, 68 percent of Americans say the US military response to the Islamic State group has not been aggressive enough. Conducted before the shootings, the poll also found that 60 percent disapprove of Obama's handling of terrorism.

Republicans have demanded that Obama back a full-scale deployment of Nato ground forces to Syria and resume controversial interrogations at the Guantanamo Bay camp, which the president wants to close.