WASHINGTON: President Barack Obama warned Americans on Sunday the terrorist threat to the United States (US) had evolved into a dangerous "new phase" in the aftermath of a deadly California shooting rampage, but vowed to destroy the militant Islamic State (IS) group and other militant groups.

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

But he offered no major shift in a strategy that has faced growing criticism it has failed to keep the country safe. Instead, in a measured way, Obama drew a careful line about what he would and would not do.

He pledged, for example, to "hunt down terrorist plotters" anywhere they are. But he insisted in the nationally televised speech: "We should not be drawn once more into a long and costly ground war in Iraq or Syria."

A White House official said on Sunday that Obama had also asked the US Department of Homeland Security and the State Department to review the K-1 fiance visa program that was used by US-born Syed Rizwan Farook, 28, and his Pakistani wife, Tashfeen Malik, 29, who were responsible for a mass shooting in California last week.

In his televised address, the US president said he had ordered a review of the "visa waiver programme" used by Tashfeen Malik to enter the US.

But a White House official confirmed Obama misspoke, meaning to say "visa programme".

The US government separately is making a series of changes to its visa waiver program to screen travellers more closely from 38 countries who can enter the US without visas.

The K-1 visa program permits foreign citizens to travel to the US to marry American citizens.

Obama spoke just four days after the couple 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". But 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 Federal Bureau of Investigation is probing 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 Nov 13 assaults in Paris that killed 130 people.

But Obama, speaking in a much less impassioned tone than French President Francois Hollande used after the Paris attacks when he vowed to make war on IS, 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, despite new questions raised about the country's defences against home-grown extremism.

"The threat from terrorism is real but we will overcome it," Obama said.

Obama's Republican critics, including the party's presidential candidates, quickly 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," Florida Senator Marco Rubio, who is seeking the Republican presidential nomination for the November 2016 election, told Fox News. "Nothing that happened in this speech tonight is going to assuage people's fears."

Digital debate looming with Silicon Valley

In a wide-ranging speech delivered from a lectern, Obama also called on Silicon Valley to help address the threat of militant groups using social media and electronic communications to plan and promote violence, setting up renewed debate over personal privacy online.

"I will urge high-tech and law enforcement leaders to make it harder for terrorists to use technology to escape from justice," Obama said.

Obama also seized the opportunity to make the case again for US gun control, something he has done to little avail because of stiff Republican resistance ─ following numerous shooting sprees during his presidency.

"We also need to make it harder for people to buy powerful assault weapons like the ones that were used in San Bernardino," Obama said. "What we can do ─ and must do ─ is make it harder for them to kill." At the same time, Obama cautioned against overreaction to the militant threat at home.

"We cannot turn against each other by letting this fight be defined as a war between America and Islam," he said, alluding to the incendiary rhetoric by Republican presidential candidates like Donald Trump, which is seen by critics as fear-mongering against the Muslim community.

Given that the California couple were not on the US national security radar before they launched their shooting spree on Wednesday, Obama faced the challenge of convincing the US public he is doing everything possible to deal with an evolving militant threat.

There was mounting evidence that the pair were "lone wolf" assailants who may have become radicalised by IS propaganda and then acted independently, making it all the more difficult for authorities to track them.

Obama's address came amid growing pressure from Republicans and even some Democrats for a tougher response to IS now that the San Bernardino shootings have raised fears among Americans about the threat of more attacks at home.

Last week's massacre, if proven to be linked to or motivated by foreign militancy, would be the deadliest such incident on US soil on Obama's watch and since the Sept 11, 2001, hijacked plane attacks on New York and Washington.

Among the few specific requests in his speech was for Congress to pass legislation that would block individuals who are on the government no-fly list from purchasing guns. That would not have stopped the California couple, however, since they were not on any government terrorism watch list.