They were said to be targeting a senior Tunisian operative
US warplanes have struck an Islamic State camp in Libya, killing as many as 40 people.
The airstrikes were targeting a senior Tunisian operative linked to two major attacks claimed by Islamic State, according to US officials.
One official, speaking on the condition of anonymity, said it is "likely" the militant was among the dead, although that is yet to be officially confirmed.
Colonel Mark Cheadle, a spokesman for the Pentagon's Africa Command, identified the operative as Noureddine Chouchane, who has been connected to Tunisia's beach massacre and the Bardo Museum attack.
"We are assessing the results of the operation and will provide additional information as and when appropriate," he said.
Local officials said the planes struck a farm house in the western city of Sabratha, near the Tunisian border, where foreign workers were living.
The city's mayor Hussein al-Dawadi said the building was completely destroyed, killing 41 people and injuring six.
Thirty-eight people, including three Irish tourists, were shot dead on a beach near the Tunisian city of Sousse in June 2015.
Three months earlier 22 people, most of them European tourists, were killed in an attack on the Bardo National Museum in Tunisia's capital Tunis.
The gunmen in both attacks, which are believed to have been linked, trained at militant camps in Libya.
Islamic State has exploited a power vacuum in Libya, taking control of the northern city of Sirte and attacking numerous oil ports.