VIDEO: Barack Obama defends strategy to combat Islamic State

US president Barack Obama has laid out the most sweeping defence yet of his strategy to defeat Islamic State (IS), but he offered no US policy shift to confront what he called a “new phase” in the terrorist threat after a mass shooting in California.

In a rare Oval Office address, Obama sought to calm a US public increasingly jittery about the fight against Islamist militancy that once appeared to be waged overseas.

His remarks failed to quiet Republican critics who have long accused him of underestimating the militants’ strength and staying power.

Speaking in a measured tone, Obama used his 14-minute nationally televised appearance to draw a careful line about what he would and would not do.

He vowed, for example, to “hunt down terrorist plotters” anywhere they are. However, he added: “We should not be drawn once more into a long and costly ground war in Iraq or Syria.”

Obama spoke days after US-born Syed Rizwan Farook, 28, and his Pakistani wife, Tashfeen Malik, 29, opened fire on a holiday party for civil servants in San Bernardino, California, killing 14 people.

The pair were killed hours later in a shootout with police.

Obama condemned the attack as “an act of terrorism designed to kill innocent people”.

However, he also said San Bernardino showed that “the terrorist threat has evolved into a new phase” as IS used the internet to “poison the minds” of potential assailants.

Obama also made a connection between national security and the need for gun control following America’s latest mass shooting.

The FBI is investigating the paramilitary-style attack in California as inspired by IS, which controls swaths of Syria and Iraq and has shown an expanded reach beyond its Middle East strongholds, including complicity in the November 13 assaults in Paris that killed 130 people.

However, Obama, whose restraint contrasted sharply with French president Francois Hollande’s impassioned words after the Paris attacks when he vowed a “merciless” response, said there was no evidence the California assault was directed by a militant group overseas or part of a broader conspiracy at home.

Nevertheless, Obama sought to show his administration was on top of the crisis. “The threat from terrorism is real but we will overcome it,” he said.

Obama’s Republican critics, including the party’s presidential candidates, panned his speech, just the third he has delivered from the Oval Office since he took office in January 2009.

“People are scared not just because of these attacks but because of a growing sense that we have a president that is completely overwhelmed by them”, said Florida senator Marco Rubio, who is seeking the Republican presidential nomination for the November 2016 election.