US President Barack Obama has said failing to prepare for the aftermath of the ousting of Libyan leader Col Muammar Gaddafi was the worst mistake of his presidency.

Obama was answering a series of questions on the highs and lows of his time in office on Fox News.

He said, however, that intervening in Libya had been "the right thing to do".

The US and other countries carried out strikes designed to protect civilians during the 2011 uprising.

But after the former Libyan president was killed, Libya plunged into chaos with militias taking over and two rival parliaments and governments forming.

So-called Islamic State (IS) gained a foothold, and Libya became a major departure point for migrants trying to reach Europe.

A UN-backed national unity government arrived in the capital Tripoli earlier this month but is waiting to take charge.

Col Gaddafi was killed during the 2011 popular uprising in Libya. Photo: Reuters

Sandy Hook 'worst day'

It is not the first time President Obama has expressed regret over Libya. He told the Atlantic magazine last month the operation went as well as he had hoped, but Libya was now "a mess".

In that interview, he also criticised France and the UK, in particular saying British Prime Minister David Cameron became "distracted" after the intervention.

President Obama told Fox that his biggest accomplishment in office was "saving the economy from the great depression".

He said the best day of his presidency was when he passed the healthcare reforms. The worst, he said, was responding to the mass shooting at Sandy Hook elementary school.

Obama discussed his legacy in a BBC interview last year, saying his failure to pass tighter gun control laws was the biggest frustration of his presidency.