The husband and wife who shot up a holiday office party this week in Southern California committed an "act of terrorism designed to kill innocent people" after "embracing a perverted interpretation of Islam," U.S. President Barack Obama said Sunday night, adding that the massacre begs for changes to U.S. weapons laws and other action from Congress.

"So far we have no evidence that they were directed by a terrorist organization overseas," Obama said in a rare live TV speech from the Oval Office. "But it is clear 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 the rampage Wednesday in San Bernardino, Calif., in which 14 people were killed and 21 wounded, epitomized a new kind of terrorist threat that has "evolved into a new phase."

"As we've become better at preventing complex, multifaceted attacks like 9/11, terrorists turn to less complicated acts of violence like the mass shootings that are becoming all too common in our society."

Obama pushes for gun safety measures0:55

He cited the rampages in 2009 at the U.S. military base in Fort Hood, Texas, and the attack in July on a military installation in Chattanooga, Tenn., as other examples. 

"As groups like ISIL grew stronger amidst the chaos of war in Iraq and then Syria, and as the internet erases the distance between countries, we see growing efforts by terrorists to poison the minds of people like the Boston Marathon bombers and the San Bernardino killers," Obama added — referring to the Islamic State of Iraq and Syria, or ISIS, by one of its alternate acronyms.

Wading into the always fractious U.S. debate over gun control, Obama challenged Congress to enact new restrictions on firearms.

"Congress should act to make sure no one on a no-fly list is able to buy a gun. What could possibly be the argument for allowing a terrorist suspect to buy semiautomatic weapons?" he said.

"We also need to make it harder for people to buy powerful assault weapons like the ones that were used in San Bernardino.... The fact is, our intelligence and law enforcement agencies, no matter how effective they are, cannot identify every would-be mass shooter, whether that individual is motivated by ISIL or some other hateful ideology.

"What we can and must do is make it harder for them to kill."

Visa program under scrutiny

Obama said he has tasked the U.S. State Department and the Department Homeland Security with re-examining the country's visa program. San Bernardino shooter Tashfeen Malik took up residence in the U.S. Malik arrived in July 2014 on a K-1 fiancée visa with her husband and co-attacker, Chicago-born Syed Rizwan Farook.

'We cannot turn against one another.' Obama warns against Islamophobia0:53

A relative of Malik's in Pakistan said the 29-year-old woman apparently became a more devoted follower of the Muslim faith in the past few years.

Hifza Batool told The Associated Press on Saturday that other relatives have said that Malik, who was her step-niece, used to wear Western clothes but began wearing the hijab head covering or the all-covering burqa donned by conservative Muslim women about three years ago.

The U.S. president said there needs to be stronger screening as well for people who don't need a visa to enter the United States, to determine whether they've travelled to war zones.  

ISIS calls California shooters 'followers'2:17

In only his third live speech from the White House's Oval Office, Obama repeated his stance that sending in ground troops to battle ISIS in Iraq or Syria would be a dangerous mistake.

"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," the president said. "They... know that if we occupy foreign lands, they can maintain insurgencies for years, killing thousands of our troops, draining our resources, and using our presence to draw new recruits." 

'A real problem that Muslims must confront'

In the final section of his 13-minute remarks, Obama appealed to the country not to lash out against American Muslims, who he said have made valuable contributions to society and to the war against terrorism. 

"That too is what groups like ISIL want. ISIL does not speak for Islam. They are thugs and killers," he said. "They account for a tiny fraction of more than a billion Muslims around the world, including millions of patriotic Muslim Americans who reject their hateful ideology. 

California Shootings

Damage to a home is shown after law enforcement raided it on Saturday evening in Riverside, Calif. The man who lives there was a neighbour of Syed Farook and the two used to fix cars together. (Chris Carlson/Associated Press)

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

However, Obama — who has drawn criticism from conservatives for not using the term "Islamic terrorism" — allowed that "an extremist ideology has spread within some Muslim communities."

"There is a real problem that Muslims must confront without excuse," he said.

Jeh Johnson, the secretary of Homeland Security, gave a hint of the speech to come in published remarks in the New York Times, indicating the U.S. administration is rethinking its strategy on fighting homegrown terrorism.

"We have moved to an entirely new phase in the global terrorist threat and in our homeland security efforts," Johnson he told the Times on Saturday. He said extremists have "in effect outsourced attempts to attack our homeland. We've seen this not just here but in other places. This requires a whole new approach, in my view."

The president's critics, and increasingly, some members of his own party, have questioned his strategy. Hours before he spoke, Hillary Clinton — his former secretary of state and the front-runner for the Democratic presidential nomination — said the U.S. is "not winning" the fight against ISIS.

After the speech, the president appeared as previously scheduled at the Kennedy Center Honors tribute in Washington.

With files from The Associated Press