TRIPOLI (AFP) - A US air strike on a militant training camp in Libya killed dozens of people on Friday, probably including a senior Islamic State in Iraq and Syria (ISIS) group operative behind attacks in Tunisia, officials said.

Pentagon press secretary Peter Cook said Noureddine Chouchane, also known as "Sabir," and other militants had been planning attacks against American and other Western interests.

"We took this action against Sabir in the training camp after determining that both he and the ISIL fighters at these facilities were planning external attacks on US and other Western interests in the region," Cook said, without providing specifics and using an alternative acronym for ISIS.

"We see what's happening in Iraq and Syria and we believe that these fighters in Libya posed a threat to our national security interests."

It was the second US air raid in the violence-wracked North African country targeting the fast-expanding militant group in the past three months.

The strike early Friday near the city of Sabratha "likely killed" ISIS operative Chouchane, a US official said earlier.

Britain's defence ministry said the strike on the camp was carried out from a Royal Air Force base, RAF Lakenheath.

"I was satisfied that its destruction makes us all safer, and I personally authorised the US use of our bases," Defence Secretary Michael Fallon said in a statement.

A militant safe house was destroyed in the dawn raid about 70km west of Tripoli, according to Hussein al-Dawadi, an official in Sabratha near the border with Tunisia.

"The raid killed 41 people who were all inside the house," Dawadi told AFP.

"The vast majority of those killed were Tunisians who were probably members of ISIS."

Cook would not confirm the figure but said a "significant" number was dead, and that as many as 60 people had been known to use the camp.

A suspect wounded in the strike said he had been taken there blindfolded for combat training, Dawadi added.

Tunisia's interior ministry issued a picture of Chouchane on Friday and a statement saying he was a "dangerous terrorist" and a wanted man.

The Sabratha Municipal Council's website said rifles, rocket-propelled grenades and other weapons were found in the house, which was rented by foreigners including Tunisians.

It posted pictures of mattresses and blankets among the rubble and a piece of metal with the inscription "Islamic State".

An AFP photographer saw four large holes he said were probably caused by missiles.

"We heard four big hits at around 3am that shook our houses," resident Moussaab Kamouka said.

Chouchane is suspected of being behind an attack in July on a beach resort near the Tunisian city of Sousse that killed 38 tourists - including 30 Britons.

He is also accused of involvement in an attack on the National Bardo Museum in Tunis in March that killed 21 tourists and a policeman.

Both assaults were claimed by ISIS, which Washington is also targeting with air strikes in Syria and Iraq where the group has proclaimed an Islamic "caliphate" and committed widespread atrocities.

The Pentagon's Cook said Washington was still assessing the raid's results.

"Destruction of the camp and Chouchane's removal will eliminate an experienced facilitator and is expected to have an immediate impact on ISIL's ability to facilitate its activities in Libya, including recruiting new ISIL members, establishing bases in Libya, and potentially planning external attacks on US interests in the region," he said.

He said the strike showed the US will target ISIS "whenever it is necessary".

In November, a US air strike in Libya killed an ISIS leader, Abu Nabil, an Iraqi also known as Wissam Najm Abd Zayd al-Zubaydi.

It was the first US strike against an ISIS leader in Libya, where the Pentagon estimates the militant group has about 5,000 fighters.

US President Barack Obama vowed Tuesday not to let ISIS build a base in Libya.

"We are working with our other coalition partners to make sure that, as we see opportunities to prevent ISIS from digging in in Libya, we take them," Obama said.

"We will continue to take actions where we got a clear operation and a clear target in mind."

ISIS has exploited the turmoil in Libya since the overthrow of dictator Moamer Kadhafi five years ago, raising fears that it is establishing a new stronghold on Europe's doorstep.

Last June, it captured the city of Sirte, 450km east of Tripoli, and has since attacked coastal oil facilities and staged a string of suicide bombings.

The internationally recognised government has been based in the country's far east, having fled a militia alliance including Islamists that overran the capital in August 2014.

The alliance has its own administration and parliament in the capital. The UN is pushing both sides to back a unity government to tackle militants and people-smugglers.