In a rare Oval Office address, US President Barack Obama vowed Sunday night the United States would overcome a terror threat that has entered a "new phase" as he sought to reassure Americans shaken by recent attacks in Paris and California.

"I know that after so much war, many Americans are asking whether we are confronted by a cancer that has no immediate cure," he said, speaking from a lectern in his West Wing office.

Obama's speech followed Wednesday's shooting in San Bernardino, California, that killed 14 people and wounded 21. Authorities say a couple carried out the attack and the wife pledged allegiance to the Islamic State and its leader in a Facebook post.

Obama said that while there was no evidence that the shooters were directed by a terror network overseas or part of a broader plot, "the two of them had gone down the dark path of radicalization."

"This was an act of terrorism designed to kill innocent people," he said in the address.

Obama announced no significant shift in US strategy and offered no new policy prescriptions for defeating the Islamic State, underscoring both his confidence in his current approach and the lack of easy options for countering the extremist group.

He did call on Congress to tighten America's visa waiver program and to pass a new authorization for military actions underway against IS in Iraq and Syria.

Many Americans are stocking up on weapons after the country's worst mass shooting in three years. Gun retailers are reporting surging sales, with customers saying they want to keep handguns and rifles at hand for self-defense in the event of another attack.

The shooting has reignited a long-running national debate over Americans' constitutional right to bear arms and whether gun ownership should be curbed or expanded as a way to stop even more bloodshed.

He also reiterated his call for tightening US gun laws, saying no matter how effective law enforcement and intelligence are, they can't identify every would-be shooter. He called it a matter of national security to prevent potential killers from getting guns.

Obama has made televised statements from the Oval Office just twice, the last in 2010 when he announced the end of combat missions in Iraq.

While Obama has spoken frequently about the Islamic State in recent news conferences and other events, the decision to speak in prime-time reflected concern among his advisers that his message isn't breaking through.

The White House has been particularly concerned about the heated rhetoric from Republican presidential candidates about Muslims.

(China Daily USA 12/07/2015 page1)