Obama acknowledges Americans' fear of terrorism but vows to overcome threat

Even so, Obama cautioned against overreaction to the terrorism 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 radicalized by Islamic State 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 Islamic State 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 Islamist militancy, would be the deadliest such incident on US soil on Obama's watch and since the Sept. 11, 2001, attacks on New York and Washington.