US President Barack Obama speaks about counter-terrorism and the United States fight against Islamic State during an address to the nation from the Oval Office of the White House in Washington, December 6, 2015. [Photo/Agencies]

WASHINGTON - US President Barack Obama vowed on Sunday to hunt down anyone plotting militant attacks against the United States as he sought to reassure Americans after a deadly California shooting rampage that has raised new questions about US defenses against homegrown extremism.

In a rare Oval Office address, Obama tried to counter mounting criticism he has not acted decisively enough to keep the United States safe from the Islamic State militant group, but he stopped short of offering any major shift in his strategy.

"The threat from terrorism is real but we will overcome it," Obama said in a nationally televised speech.

Obama spoke just four 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," but also called it a "new phase" in the fight against Islamist militancy.

The FBI is investigating the paramilitary-style attack as inspired by Islamic State, 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 said there was no evidence the assault was directed by a militant group overseas or part of a broader conspiracy at home.

The Obama administration plans to seek greater cooperation from US technology companies to help ferret out such apparently homegrown attack plots, which could rekindle a privacy-versus-security debate between the government and Silicon Valley.