WASHINGTON (Reuters) - More than 80 Islamic State militants, some of whom were believed to be plotting attacks in Europe, died in U.S. air strikes on camps outside the group’s former North African stronghold of Sirte in Libya, the United States said on Thursday.

“These strikes were directed against some of ISIL’s external plotters,” U.S Defense Secretary Ash Carter told a Pentagon briefing, using an acronym for Islamic State, the Syria- and Iraq-based militant group.

“They 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,” Carter said.

The U.S. attack on Wednesday night was led by two B-2 bombers, which took off from Whiteman Air Force Base in Missouri to carry out a 30-hour round-trip bombing run. They dropped about 100 precision-guided munitions on the camps.

The attack took place 28 miles (45 km) southwest of Sirte, targeting fighters fleeing Libyan forces clearing them from Sirte with support from U.S. air strikes.

Libyan forces finished retaking the city in early December after a nearly seven-month battle in a major blow to Islamic State that left the group without any territory in Libya.

Still, Libyan officials say hundreds of militants may have escaped before the battle or in its early stages, and local Islamic State leaders have spoken of trying to regroup outside the city.

A statement from Libya’s U.N.-backed Government of National Accord (GNA) in Tripoli said the strikes were “preemptive steps to foil attempts” by Islamic State to stage new attacks on Sirte as well as other areas of the country.

The strikes appeared to be intended to shore up the GNA and protect recent gains in oil production from possible militant attacks, said Geoff Porter, head of North Africa Risk Consulting.

The GNA has received strong backing from the outgoing U.S. administration but has struggled to win support inside Libya or reverse the chaos that developed after rebels overthrew Muammar Gaddafi in 2011 with help from NATO.

It’s unclear whether Donald Trump, who takes office as U.S. president on Friday, might seek to alter U.S. policy in Libya.

The United States carried out nearly 500 air strikes against Islamic State targets in Sirte between August and December. It had previously carried out several isolated strikes against militant targets elsewhere in Libya.

The jihadist group took over Sirte in early 2015, turning it into its most important base outside the Middle East and attracting large numbers of foreign fighters to the city. It imposed its ultra-hardline rule on residents and extended its control along about 155 miles (250 km) of Libya’s Mediterranean coastline.