US stealth bombers kill 80 IS militants in Libya

American B-2 stealth bombers and armed drones have attacked two Islamic State military camps in Libya, killing 80 IS militants near their former bastion of Sirte, Pentagon chief Ash Carter said on Thursday. 

"They certainly are people who were actively plotting operations in Europe and may also have been connected to some attacks that have already occurred in Europe," Carter said on his last day in office.

The strike late Wednesday came a month after the United States officially wrapped up military operations in and around Sirte, where it had conducted nearly 500 strikes to help the Libyan unity government expel militants from the coastal city.

The fighters "were seen immediately beforehand carrying weapons, wearing tactical vests, carrying mortars and standing in formation," a Pentagon official told AFP, speaking on condition of anonymity.

He added that no civilians were thought to have been killed and no women or children were present.

The camps were about 45 kilometres south-west of Sirte. 

Pentagon spokesman Peter Cook said those targeted included IS fighters who had previously fled Sirte. 

"They posed a security threat to Libya, the region and US national interests," Cook said in a statement, noting that the strikes appeared to have been "successful".

The Pentagon had launched Operation Odyssey Lightning to help local forces push IS from Sirte on 1 August last year.

When operations concluded last month, following the city's "liberation," the Pentagon left open the possibility of conducting additional anti-IS attacks if Libya's Government of National Accord (GNA) asked for help in doing so.

Wednesday's strike was conducted with the "cooperation" of the GNA, the defence official said, noting that the move had been authorised by outgoing President Barack Obama.

B-2 stealth bombers, which have a distinctive flying-wing design, conducted the strike with the help of armed drones.

The fall of Sirte - former leader Muammar Gaddafi's hometown located 450 kilometres east of Tripoli - was a major setback for IS, which has also faced military defeats in Syria and Iraq.

Libya descended into lawlessness after the NATO-backed ousting of longtime dictator Gaddafi in 2011, with rival administrations emerging and well-armed militias vying for control of its vast oil wealth.

"These strikes will degrade ISIL's ability to stage attacks against Libyan forces and civilians working to stabilise Sirte, and demonstrate our resolve in countering the threat posed by ISIL to Libya, the United States and our allies," Cook said, using an alternate IS acronym.

President-elect Donald Trump's position on Libya is unclear and his public statements have reflected shifting views.

In 2011, he urged foreign military intervention to topple Gaddafi.

"We should do on a humanitarian basis, immediately go into Libya, knock this guy out very quickly, very surgically, very effectively, and save the lives," he said.

Then in 2015, he said the world would be "100 percent" better off if Saddam Hussein in Iraq and Gaddafi in Libya were still in power, adding that human rights abuses are "worse than they ever were" in the two countries.