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]

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.

The President also asked the US Department of Homeland Security and the State Department to review the K-1 fiance visa program used by the couple responsible for the shooting, saying that he had ordered a review of the "visa waiver program" used by Tashfeen Malik to enter the United States. The K-1 visa program permits foreign citizens to travel to the United States to marry American citizens.

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.