It was the third time in 10 days that Obama addressed America's counter-terrorism efforts in the wake of the mass shooting in California that is being investigated as a terror attack.
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"At this moment, our intelligence and counterterrorism professionals do not have any specific and credible information about an attack on the homeland," the president said after meeting top US security officials at the National Counterterrorism Center in Virginia.
"That said, we have to be vigilant," he said. "As I indicated in my address to the nation last week, we are in a new phase of terrorism including lone actors and small groups of terrorists like those in San Bernardino.
"Because they are smaller, often self-initiating, self-motivating, they're harder to detect. And that makes it harder to prevent. But just as the threat evolves, so do we."
The December 2 murder of 14 people in San Bernardino was carried out by US-born Syed Farook and his Pakistani wife Tashfeen Malik. The radicalized Muslim couple are believed to have been inspired, if not directed, by the Islamic State group -- illustrating the evolving threat posed by the jihadist network.
Obama repeated his pledge to hunt down Islamic State in its stronghold in Syria, where the United States is leading a coalition bombing the jihadists, and to do more to prevent battle-hardened fighters returning to the West to stage attacks.
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"We're sending a message; if you target America, you will have no safe haven," he said. "We will find you, and we will defend our nation," he said.
"We're doing more with countries around the world, including our European partners, to prevent the flow of foreign terrorist fighters both to places like Syria and Iraq and back into our countries."
Obama has also ordered a review of the K-1 visa system, also known as the "fiancee visa", which Malik used to enter the United States. Possible changes include increased monitoring of social media posts and a more detailed background check and interview process.
Authorities also want to add security steps to America's popular visa-waiver program, under which visitors from dozens of friendly countries can come without a visa.
"Since 9/11 we've taken extraordinary steps to strengthen our homeland security," Obama said. "Our borders, our ports, our airports, our aviation security including enhanced watch lists and screening."
After two decades battling militants, Americans appear increasingly divided on the nature of the problem posed by Islamic State and how to respond.
According to a recent CNN/ORC poll, 68 percent of Americans say the US military response to the Islamic State group has not been aggressive enough.
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.
Conservatives have also taken issue with Obama's refusal to use the phrase "radical Islam" which the White House says would confer on terrorists the legitimacy of a faith they have betrayed.
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Obama indirectly addressed the heated rhetoric from Republican presidential candidates on national security, chief among them Donald Trump, who sparked global outrage with a call to close US border to Muslims.
"When Americans stand together, nothing can beat us," Obama said.
"We cannot give in to fear or change how we live our lives, because that's what terrorists want. That's the only leverage that they have. They can't defeat us on a battlefield, but they can lead us to change in ways that would undermine what this country's all about."
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