US planes bombed Islamic State targets in Libya on Monday, responding to the UN-backed government's request to help push the militants from their former stronghold in the city of Sirte.
"The first air strikes were carried out at specific locations in Sirte today causing severe losses to enemy ranks," Prime Minster Fayez Seraj said on state TV.
Forces allied with Seraj have been battling Islamic State in Sirte - the home town of former dictator Muammar Gaddafi - since May.
The group seized the Mediterranean coastal city last year, making it its most important base outside Syria and Iraq, but its militants are now besieged in a few square kilometres of the centre where they hold strategic sites including the Ouagadougou conference hall, the central hospital and the university.
The last acknowledged US air strikes in Libya were on an Islamic State training camp in the western city of Sabratha in February.