President Barack Obama urged Americans Sunday to reject divisiveness and vilifying Islam even as he confirmed that radical jihadis had massacred 14 people last week in a terrorist attack in California.

In a sombre Oval Office speech that ruled out sending ground troops to Syria and called for tougher gun control at home, Mr. Obama said there was "no immediate cure" to the cancer of terrorism.

"Success won't depend on tough talk, or abandoning our values, or giving in to fear," he said, seeking to reassure the country that its principles would triumph over the siren song of radical extremism.

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"Never forget what makes us exceptional," the President said in a 13-minute speech. He reminded Americans that many Muslims serve in the U.S. military and that Muslim communities across the United States have condemned the husband-and-wife team who, clad in combat gear, stormed a holiday party in San Bernardino with assault rifles last week.

"That kind of divisiveness, that betrayal of our values plays into the hands of groups like ISIL," Mr. Obama said, referring to Islamic State by its older name, Islamic State of Iraq and the Levant. The Pakistani-born wife in the jihadi couple apparently pledged allegiance to Islamic State in a Facebook posting shortly before leaving her six-month-old child with a relative as the couple launched the attack.

Mr. Obama said there was "no evidence that the killers were directed by a terrorist organization overseas" but counterterrorist agencies in the United States continued to probe for links. Still, the President confirmed: "This was an act of terrorism" by the U.S.-born Syed Rizwan Farook, 28 and his wife, Tashfeen Malik, 29, who arrived in the United States on a fiancée visa in 2014.

The war against Islamic extremist will remain multipronged, Mr. Obama said.

He praised America's "closest allies, including France, Germany and the United Kingdom" for stepping up the bombing campaign in the U.S.-led air war against Islamic State. Mr. Obama made no mention that the new Canadian government, led by Prime Minister Justin Trudeau, had bailed out of the air strikes.

He disparaged Islamic State, saying the group "does not speak for Islam; they are thugs and killers."

But despite the rising cries to escalate, Mr. Obama said he would "not be drawn into a long and costly ground war." The President, who won in 2008 on his promise to pull U.S. troops out of Iraq, said it was counterproductive to "send a new generations of Americans overseas to fight and die on foreign soil."

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At home, the President renewed his often-ignored call for tougher gun laws.

"Congress should act to make sure no one on a no-fly list is able to buy a gun … it's a matter of national security," he said, adding that he wanted to make it harder for people to buy high-powered military assault weapons like the ones used in the San Bernardino attack.

Last week's California massacre in California was the second major terrorist attack inspired by Islamic extremism inside the United States on Mr. Obama's watch.

In 2009, a U.S. army psychiatrist shouting "Allahu akbar," or "God is Great," killed 13 people and injured more than 30 when he attacked soldiers at Fort Hood, Tex. Major Nidal Hasan, paralyzed by a bullet from the civilian police woman who shot him, was eventually convicted of murder. Maj. Hasan was a follower of Anwar al-Awlaki, the Virginia-born U.S. citizen who became a radical Islamic cleric and al-Qaeda leader and communicated with Maj. Hassan before the Fort Hood atrocity.

Mr. Obama subsequently ordered the killing in Yemen – by Hellfire missile launched from a Predator drone – of Mr. al-Awlaki, the first known presidentially sanctioned execution of a U.S. citizen away from a battlefield and without trial.

The San Bernardino attack, claimed by Islamic State, followed a deadly assault on multiple locations in Paris by Islamic jihadis last month that killed 130 people and the bombing of a Russian passenger jet over the Sinai Peninsula that killed all 224 people on board.

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Russia and France escalated their bombing campaigns. Mr. Obama has resisted escalating the air war.

Even before he spoke – in only his third Oval Office televised address to the nation – Mr. Obama's critics were savaging him, claiming the President was both weak and indecisive, that he created the vacuum from which Islamic State emerged by insisting on pulling all U.S. troops out of Iraq and that his dithering on Syria had spurred the spiral into violence that has destroyed the country, forcing millions to flee while President Bashar al-Assad remains in power.

Mr. Obama has repeatedly refused to put significant numbers of U.S. troops on the ground or scale up the year-long bombing campaign which has, so far, failed to defeat Islamic State or tip the balance in the Syrian civil war.

Republicans – and even Hillary Clinton, the President's former secretary of state and Democratic front-runner for the 2016 election – accuse Mr. Obama of not doing enough.

The U.S. is "not winning" the war against Islamic State, Ms. Clinton said Sunday, adding: it was "too soon to say that we are doing everything we need to do." She has called for enforcing a no-fly zone over Syria.

Republicans were less measured in their demands that Mr. Obama escalate the war. "We will carpet bomb them into oblivion," Senator Ted Cruz of Texas said while campaigning Sunday in Iowa. "I don't know if sand can glow in the dark, but we're going to find out."

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Donald Trump, the Republican front-runner, has called for forced registration of millions of Muslims – including citizens – in the United States.

Senator Marco Rubio of Florida has called for a Sunni Arab army backed by U.S. air power to defeat Islamic State. "We have a President who leaves our allies feeling betrayed and our adversaries feeling emboldened," he said.

Senator Lindsey Graham of South Carolina said Mr. Obama's failure to defeat Islamic State was setting the stage for a "9-11" scale attack. It's only "a matter of time until we get hit by ISIL," he said, using an older acronym for the militants who have carved out a nascent caliphate in western Iraq and parts of Syria. "Not some lone-wolf attack but hardened terrorists are coming here to hit us hard if we don't hit them first," Mr. Graham said Sunday.