December 7, 2015 1:40 am

President Barack Obama told the nation on Sunday that the recent shooting of 14 victims in California was an “act of terrorism”, as he warned that the threat from terrorism had “evolved into a new phase”.

Speaking from the Oval Office in a rare prime time address, Mr Obama said there was no evidence that the couple suspected of the massacre had belonged to a terror group, but that it was clear they had “embraced a perverted interpretation of Islam that calls for war against America and the West”.

In a speech designed to counter criticism that he was not doing enough to tackle terrorism amid the rise of Isis, particularly following the recent attacks in Paris, Mr Obama said that the US would “prevail by being strong and smart”.“The threat from terrorism is real, but we will overcome it. We will destroy Isil [an alternative name for Isis] and any other organisation that tries to harm us,” said Mr Obama. “Our success won’t depend on tough talk, or abandoning our values or giving into fear. That’s what groups like Isil are hoping for. Instead, we will prevail by being strong and smart, resilient and relentless.”

Rejecting calls from some Republicans to send large numbers of troops to Syria and Iraq to combat Isis, Mr Obama said the US “should not be drawn once more into a long and costly ground war” in those countries.

“That’s what groups like Isil want. They know they can’t defeat us on the battlefield . . . but 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.”

Mr Obama said the US strategy of using air strikes, small numbers of special fighters and working with local forces in Iraq and Syria would “achieve a more sustainable victory” without “sending a new generation of Americans overseas to fight and die for another decade on foreign soil”.

Although the FBI previously said that the shootings were an act of terrorism, officials have urged people not to draw firm conclusions about the case. “I would caution people not try and define either of these two individuals right now,” attorney-general Loretta Lynch said on Sunday.

FBI director James Comey told reporters not to make too much of the fact that the two suspects had been in contact with people under investigation by the bureau, in an indication that those contacts could have been superficial. Investigators are still combing through every detail of the lives of America-born Syed Rizwan Farook and his Pakistani wife Tashfeen Malik.

Despite the caveats, the incident has already taken on some of the hallmarks of what is the worst nightmare for counter-terrorism officials — a US citizen whose descent into Jihadi radicalism went almost totally undetected and who was willing to attack the most vulnerable of targets.

As Republicans such as Donald Trump urge Muslims to spy on their neighbours, Mr Obama on Sunday encouraged the American people not to succumb to the politics of fear, while calling on American Muslims to help the country beat back terrorism.

“We cannot turn against one another by letting this fight be defined as a war between America and Islam. That, too, is what groups like Isil want. Isil does not speak for Islam. They are thugs and killers, part of a cult of death,” said Mr Obama.

However, he said that his call for reason “does not mean denying the fact that an extremist ideology has spread within some Muslim communities”, adding that it was a “real problem that Muslims must confront without excuse”.

As Isis has managed to attract thousands of recruits from around the world over the past 18 months, US officials have taken a certain amount of comfort from what they see as the greater capacity of American society to absorb Muslim immigrants, compared to western Europe.

But the San Bernardino shooting and its potential link to Isis has provided a stark reminder that the US is also vulnerable to homegrown Jihadi violence, including the sort of lone wolves whose self-radicalisation is desperately hard for law enforcement agencies to detect.

Paul Ryan, the new Republican Speaker of the House of Representatives, said the events of recent weeks underscored that US efforts to defeat Isis had been a “failure”.

“Until we hear from the president what more can be done — with our military, our intelligence-gathering, and our international partners — we will remain one step behind our enemy,” said Mr Ryan. “This is not just the next president’s problem. It is our problem, and we must confront it today.”

Mr Comey has said that US law enforcement agencies are conducting more than 900 investigations in 50 states into potential Isis sympathisers. US officials have said that around 250 Americans have attempted to travel to Syria or Iraq to join Isis — although this number pales beside the tens of thousands who tried to make the trip from Western Europe.

According to a report released last week by researchers at George Washington University, US law enforcement have charged 71 people since March 2014 for Isis-related activities.

Although the US has seen cases in the past of individuals trying to get involved with Islamist extremist groups, Lorenzo Vidino, director of the university’s centre on extremism, said that the scale of “Isis-related mobilisation and radicalisation is unprecedented”.

One of the problems for agencies tracking the growing support for Isis is that the motivations of the individuals defies easy analysis because they come from such diverse backgrounds in terms of social class, education and geography. “We have seen cases in big cities and rural towns,” he said.

“The individuals involved range from hardened militants to teenage girls, petty criminals and college students.”

The GW research showed that there were around 300 Isis sympathisers based in the US who were active on social media — Twitter being the preferred platform — where they spread propaganda for the group and made contact with other potential individuals with similar opinions. The report said that while there were many cases where radicalisation took place through online interactions, in many other instances face-to-face relationships were also essential in promoting Isis narratives.

The average age of the Isis supporters in the US was 26, the report said, while the youngest person to be arrested was an unnamed 15 year-old boy. Of the people arrested, 14 per cent were women — another unprecedented factor in the support for Isis.