WASHINGTON (BLOOMBERG) - A failure to adequately plan for the aid and governing of Libya after the US-led Nato attacks in 2011 "probably" was his biggest error in office, President Barack Obama said in an interview on "Fox News Sunday."

Asked by host Chris Wallace about the "worst mistake" of his soon-to-end White House years, Obama listed the aftermath of the ouster and death of Moammar Qaddafi, even as he defended the intervention.

"Probably failing to plan for the day after," Obama said in the session, which was taped at the University of Chicago on April 7. He added that intervening in Libya "was the right thing to do."

The Libya operation and its chaotic aftermath has been resurrected in the 2016 presidential campaign. That's in part because of the increasing presence of the Islamic State there, and US airstrikes to disrupt its operation.

Democratic contender Hillary Clinton, as Obama's secretary of state, strongly supported the intervention. In a 2011 interview with CBS News when still secretary, Clinton said of Qaddafi, "We came. We saw. He died."

'DEEPLY REGRETTABLE'

At a March 7 town hall meeting, Clinton said what has happened since then "is deeply regrettable. There have been forces coming from the outside, internal squabbles that have led to the instability that has given terrorist groups, including ISIS, a foothold in some parts of Libya."

"I think it's fair to say, however, if there had not been" an intervention "we would be looking at something much more resembling Syria now, than what we faced in Libya," she said in March.

Army General David Rodriguez, head of US Africa Command, told reporters last week that the Islamic State presence in Libya has doubled since 2015 to as many as 6,000 fighters.

Former Defense Secretary Robert Gates told Yahoo News in January that he thought Clinton's "influence was pivotal in persuading the president to broaden the goal in Libya beyond just saving the people in Benghazi" from Qaddafi's forces and "essentially focusing more on regime change. The president told me that it was one of the closest decisions he'd ever made, sort of 51-49, and I'm not sure that he would've made that decision if Secretary Clinton hadn't supported it."

'POLITICAL TRANSITION'

The Congressional Research Service this month wrote that Libya's "political transition has been disrupted by armed non- state groups and threatened by the indecision and infighting of interim leaders" after the armed uprising toppled Qaddafi's regime.

"Interim authorities" have "proved unable to form a stable government, address pressing security issues, reshape the country's public finances, or create a viable framework for post-conflict justice and reconciliation," according to CRS, the public policy research arm of the US Congress.

A United Nations-sponsored unity government led by Fayez Serraj assumed office last month and has won support from politicians and militias, offering hope that Libya may begin to emerge from the turmoil that has uprooted nearly half a million people since Qaddafi fell in 2011 after more than four decades in power.

Obama told Fox News that his best day in office was the one on which his signature health care plan was passed, and his worst was "the day we travelLed up to Newtown," after the shooting deaths of 20 children and six adult staff members at an elementary school in Sandy Hook, Connecticut.