The strikes targeted positions in the port city of Sirte, an IS stronghold.
Libyan PM Fayez al-Sarraj, in a televised addressed, said the strikes caused “heavy losses”.
Western powers have become increasingly concerned at Islamic State’s growing presence in Libya.
The air strikes are the first such US military intervention co-ordinated with the Libyan unity government.
The Pentagon said the strikes, authorised by President Barack Obama, were in support of government forces currently fighting IS militants.
The government began an offensive against IS fighters in May and said two weeks ago that it had made its largest gains to date.
“These actions and those we have taken previously will help deny ISIL a safe haven in Libya from which it could attack the United States and our allies,” the Pentagon statement continued, using another term for IS.