US warplanes carried out air strikes early on Friday morning in the western Libyan city of Sabratha, where Islamic State (IS) militants allegedly operate, killing as many as 40 people.

A US military spokesman said the attacks targeted a senior Tunisian militant linked to attacks in Tunisia last year. Sabratha's mayor Hussein al-Thwadi told Reuters the planes struck at 3.30am local time, hitting a building in the Qasr Talil district in which foreign workers were living. He said 41 people had been killed and six wounded. The death toll could not immediately be confirmed with other officials.

Sabratha lies about 50 miles west of Tripoli near the Tunisian border, and is one of the areas where Western officials say Islamic State (IS) militants had some presence as part of their expansion in the North African state.

Two major attacks in Tunisia last year claimed by IS — one on a Sousse resort hotel and another on a Tunis museum — were carried out by gunmen who officials said had trained in Libya.

Tunisian security sources have said they believe Tunisian IS fighters have been trained in camps near Sabratha.

The New York Times earlier reported that Friday's air strikes targeted a senior Tunisian operative, Noureddine Chouchane, connected to both of last year's attacks.

The mayor said officials visited the site of the strike and found weapons in the building, but he did not give any further details. Some Tunisians, a Jordanian and two women were among the dead, he said.

Several Tunisians who had recently arrived in Sabratha were among survivors.

Since Libyan leader Muammar Qaddafi was overthrown in 2011, the north African country has slipped deeper into chaos with two rival governments each backed by competing factions of former rebel brigades.

As IS militants have expanded their reach in Libya, taking over the city of Sirte and attacking oil ports, so too have calls increased for a swift Western response to stop the group establishing a base outside its Iraq and Syria territory.

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

US and European officials insist Libyans must invite help through a united government, but say they may still carry out unilateral action if needed.

Last November the US said it carried out an air strike on Libya's Derna to target Abu Nabil, also known as Wissam Najm Abd Zayd al Zubaydi, an Iraqi IS commander.

Watch the VICE News documentary: **Libya's Quiet War: The Tuareg of South Libya:**