Washington (AFP) - More than 80 Islamic State jihadists were killed in a US aerial blitz on training camps in Libya, including fighters involved in plotting attacks in Europe, Defense Secretary Ashton Carter said Thursday.

The Pentagon made the highly unusual decision to conduct the air strikes with a pair of B-2 stealth bombers that flew to North Africa on a 34-hour mission from their base in Missouri in America's Midwest.

The last time the distinctive, bat-shaped planes were used in Libya was in 2011 during the mission that led to the ouster of longtime leader Moamer Kadhafi.

Wednesday's massive strike saw the B-2s and Reaper drones unleash about 100 bombs on the IS training camps -- equating to more than one bomb per jihadi that was killed.

The camps were located about 45 kilometers (30 miles) southwest of the coastal city of Sirte, Kadhafi's former home town that IS for a time turned into a stronghold as it attempted to expand its presence in Libya.

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

The air assault came a month after the United States had officially wrapped up its military operations in and around Sirte.

The Pentagon launched that mission, Operation Odyssey Lightning, on August 1 and it comprised about 500 strikes.

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

Wednesday's strike was conducted in full coordination with the GNA, Pentagon spokesman Peter Cook said, noting the operation had been authorized by outgoing President Barack Obama.

He displayed brief footage taken ahead of the strike showing a group of men standing by two camouflaged pick-up trucks and unloading what appeared to be bombs or rockets.

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Officials said no civilians were thought to have been killed and no women or children were present during the massive strike.

- 15 air tankers -

Cook would not discuss why the Air Force chose to use the B-2s, or whether it was a show of force as Obama leaves the White House ahead of President-elect Donald Trump's inauguration on Friday.

"The use of the B-2 demonstrates the capability of the United States to deliver decisive precision force to the Air Force's Global Strike Command over a great distance," Cook said.

The United States has 20 B-2 bombers and the Air Force needed to fly 15 air tankers to ensure refueling over the course of the lengthy mission, Air Force spokesman Colonel Pat Ryder said.

The fall of Sirte, located 450 kilometers (280 miles) east of Tripoli, was a major setback for IS, which has also faced military defeats across Syria and Iraq.

Libya descended into lawlessness after the NATO-backed ousting of longtime dictator Kadhafi 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 stabilize 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.

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

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

"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 Kadhafi in Libya were still in power, adding that human rights abuses are "worse than they ever were" in the two countries.