A US air strike in Libya believed to have killed an Islamic State group operative behind the mass murder of tourists on a Tunisian beach also killed two kidnapped Serbian embassy employees, Belgrade said Saturday.

The Friday attack, which has been condemned by Libya's internationally recognised government, targeted a jihadist training camp near the Libyan coastal city of Sabratha, killing dozens of people.

But Belgrade said the victims of the strike also included two employees from its embassy in Libya, who were taken hostage on November 8 in Sabratha from a convoy of cars heading to the Tunisian border.

US officials said the raid likely killed Noureddine Chouchane, also known as "Sabir," who along with other jihadists had been planning attacks against American and other Western interests.

Chouchane is suspected of being behind both the beach attack in July 2015 near the Tunisian city of Sousse that killed 38 tourists, including 30 Britons, and an attack on the National Bardo Museum in Tunis that killed 21 tourists and a policeman in March 2015.

Both attacks were claimed by the Islamic State group.

"It has just been officially confirmed that two Serbian citizens who were foreign ministry employees, Sladjana Stankovic and Jovica Stepic, were killed," Serbian Prime Minister Aleksandar Vucic told reporters.

"They were killed by explosions, obviously we are talking about American bombs," he said, expressing "deepest condolences" to the families.

"This is the first big hostage crisis that our state has been faced with. Our people would have been released had they not been killed," insisted Vucic.

The Pentagon for its part said it had "no information" indicating that its air strike had killed the two Serbs and that the circumstances of their deaths "remained unclear".

"Our forces watched this training camp for weeks leading up to the operation, and at the time of the strike there were no indications of any civilians present," spokesman Peter Cook said, extending his condolences to the Serbian government and the victims' relatives.

Libya's internationally recognised government in the far east of the country issued a statement saying the US attack was not coordinated with the authorities and was "a clear and flagrant violation of sovereignty of the Libyan state".

The Serbian premier said the bodies of Stankovic, the embassy's communications chief, and her driver Stepic were expected to be repatriated on Monday.

Sabratha, which lies 70 kilometres (42 miles) west of Tripoli, is considered a stronghold of extremism in lawless Libya, where militants are trained in jihadist camps before launching deadly attacks in other countries.

The country spiralled into chaos after longtime dictator Moamer Kadhafi was ousted and killed in October 2011, with two governments vying for power and armed groups battling to control vast energy resources.

Belgrade maintains an embassy in Tripoli, and Serbian citizens -- mostly doctors, other medical staff and construction workers -- have been working in Libya for decades due to close bilateral relations during Kadhafi's regime.

- IS training in Libya -

The Pentagon estimates that around 5,000 IS fighters are in Libya, and Friday's strike was the second US air raid in the past three months targeting the fast-expanding group in the North African country.

A statement from the Tripoli-based general prosecutor said that one of those wounded in the US strike had confirmed that he and the others killed in the raid were IS members.

The wounded man said they "came to Libya to train and then carry out terrorist attacks in Tunis", the prosecutor's statement said.

Vucic said Washington was probably unaware that the Serbian hostages were held at the bombed location, "but it will remain unknown".

He said relations with the United States were "historically not easy", referring to the 11-week bombing campaign by NATO against Serbia in 1999 during the Kosovo war.

"I am not ready... to strain and worsen relations with the United States because we need them as a friend and partner, not as an enemy," he added.

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