Obama admits challenging to spot ‘lone wolf’ domestic threats and stop Islamic State

US President Barack Obama admitted that “lone wolf” jihadist attacks present a vexing new security challenge Friday, even as he assured Americans the Islamic State group can be defeated.

Obama capped a week in which he has tried repeatedly to reassure a jittery Americans that his administration can neutralise the terror threat, with an end-of-year press conference that only underscored the tough task ahead.

“It is very difficult for us to detect lone wolf plots,” Obama acknowledged as officials continue to piece together how a couple shot 14 people dead at a party in California.

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“Despite the incredible vigilance and professionalism of all our law enforcement ... you don’t always see it.”

“This is a different kind of challenge than the sort that we had with an organisation like al-Qaeda,” Obama said before leaving for San Bernardino, the site of the shooting, and on to Hawaii for Christmas.

“Essentially, you have ISIL trying to encourage or induce somebody who may be prey to this kind of propaganda.”

Obama is under intense pressure to quickly and decisively end a threat that may take years if not decades of military, security, diplomatic and economic effort to neutralise.

Part of the puzzle, he said, was to end the brutal war in Syria that has created fertile soil for the Islamic State to grow.