US warplanes launched air strikes against a suspected Islamic State training camp in western Libya on Friday, killing more than 40 people, likely including a militant connected to two deadly attacks last year in neighbouring Tunisia.

It was the second US air strike in three months against Islamic State in Libya, where the hardline Islamist militants have exploited years of chaos following Muammar Gaddafi's 2011 overthrow to build up a presence on the southern shores of the Mediterranean Sea.

The Pentagon said it had targeted an Islamic State training camp. The facility in the city of Sabratha was linked to Noureddine Chouchane, a Tunisian blamed by his native country for attacks last year on a Tunis museum and the Sousse beach resort, which killed dozens of tourists.

"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," the Pentagon said, using an acronym for Islamic State, also known as ISIS or Daesh.

US officials said Chouchane is most likely dead but White House spokesman Josh Earnest said he could not yet confirm the results of the air assault. He said the raid showed US willingness to fight Islamic State.

"It's an indication that the president will not hesitate to take these kinds of forceful, decisive actions," Earnest said.

In Libya, photos released by the municipal authorities showed a massive crater in grey earth. Several wounded men lay bandaged in hospital.

The mayor of Sabratha, Hussein al-Thwadi, told Reuters the planes hit a building in the city's Qasr Talil district, home to many foreigners.

Locals officials said 43 people were killed.

The strikes targeted a house in a residential district west of the centre, municipal authorities said in a statement.

The house had been rented to foreigners including Tunisians suspected of belonging to Islamic State, and medium-calibre weapons including machineguns and rocket-propelled grenades had been found in the rubble, the statement said.

The air strikes came just days after a warning by President Barack Obama that Washington intended to "take actions where we've got a clear operation and a clear target in mind" against Islamic State.

Britain said it had authorised the use of its airbases to launch the attack.

Islamic State runs a self-styled caliphate across swathes of Iraq and Syria, where it has faced air strikes from a US-led coalition since 2014.

DEEPER INTO CHAOS

Since Gaddafi was overthrown five years ago by rebel forces backed by NATO air strikes, Libya has slipped deeper into chaos, with two rival governments each backed by competing factions of former rebel brigades.

A U.N.-backed government of national accord is trying to win support, but is still awaiting parliamentary approval. It is opposed by factional hardliners and has yet to establish itself in the capital Tripoli.

Islamic State has expanded, attacking oil ports and taking over Gaddafi's home city of Sirte, now the militant group's most important stronghold outside its main redoubts in Syria and Iraq.

Calls have increased for a swift Western response to stop the group establishing itself more permanently and using Libya as a base for attacks on neighbours Tunisia and Egypt.

Western officials and diplomats have said air strikes and special forces operations are possible as well as an Italian-led "security stabilisation" plan of training and advising.

US and European officials have in the past insisted Libyans must first form a united government and ask for help, but they also say they may still carry out unilateral action if needed.

The United States estimates that the number of militants directly affiliated with Islamic State or sympathetic to it now operating in Libya is in the "low thousands," or less than 5,000, a US government source said.

Last November the United States carried out an air strike on the Libyan town of Derna, close to the Egyptian border, to kill Abu Nabil, an Iraqi commander in Islamic State.

A US air strike targeted veteran Algerian militant Mokhtar Belmokhtar and other jihadists meeting in eastern Libya last June. His fate is unclear. – Reuters