WASHINGTON: President Barack Obama urged Americans to remain vigilant against the potential threat of homegrown Islamic State (IS) militants, on Friday, due to the difficulty of tracking “lone wolf” attackers like those who went on a shooting spree in California.

Obama appeared in the White House press briefing room for a year-end news conference shortly before traveling to San Bernardino, California, where the Dec. 2 shootings took place, to meet privately with families of the victims en route to spending the holidays in Hawaii.

Obama talked tough about the prospects of defeating IS militants who control broad swathes of territory in Syria and Iraq but admitted United States (US)' law enforcement agencies have limitations in tracking the threat at home.

“All of us can do our part by staying vigilant, by saying something if we see something that is suspicious, by refusing to be terrorised and by staying united as one American family,” Obama said.

A day after telling Americans that there is no current credible militant threat in the US, Obama acknowledged that “lone wolf plotters” are difficult to track, particularly if they are a husband and wife like the two who killed 14 people in San Bernardino.

As for US efforts to track potential attackers, Obama said social media postings by potential militant suspects are constantly being reviewed by law enforcement agencies but that private communications are far more difficult to track.

He said he believes law enforcement officials have struck the right balance between privacy concerns and making sure information gathering is carried out.

Turning to a festering foreign policy dilemma, Obama trod carefully around the subject of how to get Syrian President Bashar al-Assad to leave power.

Obama for years has demanded Assad resign, but no political transition has ever been agreed upon and Russia's move to defend Assad has complicated diplomatic efforts to find a way for Assad to give up power and resolve the country's civil war.

Obama has drawn fire on his approach to Syria from Republican presidential candidates who say he has left a leadership vacuum that Russian President Vladimir Putin has sought to fill.

“I think that Assad is going to have to leave in order for the country to stop the bloodletting and for all the parties to be able to move forward in a non-sectarian way. He has lost legitimacy,” Obama said.

He said diplomats need to find a way to create a political transition that allows Assad's allies like Russia and Iran to ensure “that their equities are respected” in Syria.

On the US campaign race to succeed him, Obama confidently predicted a Democrat will succeed him when he leaves office in January 2017.

The president's year-end session with reporters was buoyed by what he considered solid global accomplishments on climate change, trade deals, a nuclear deal with Iran, and at home, an improving economy and a just-completed budget deal with Congress.

As for next year, Obama described a “handful of areas” where progress could be made in 2016, even though the legislative process would be “skewed” by election-year politics.

He cited a Pacific trade pact and criminal justice reform as examples.

Obama said his administration is working systematically to reduce the detainee population at the US naval base at Guantanamo Bay, Cuba, a detention facility he has set a goal to close.

Obama said he would attempt to get the Republican-led Congress to back a plan to close the facility before he considers whether to use his executive authority to address the issue. Many Republicans strongly oppose closing the site.

“I'm not going to automatically assume that Congress says no” to his proposal to close the facility, he said.