Planes bombed targets responding to the U.N.-backed government’s request to help push the militants from their former stronghold of Sirte.
“The first air strikes were carried out at specific locations in Sirte causing severe losses to enemy ranks,” Prime Minster Fayez Seraj said on state TV.
Pentagon spokesman Peter Cook said the strikes did not have “an end point at this particular moment in time”.
Forces allied with Seraj have been battling Islamic State in Sirte – the home town of former dictator Muammar Gaddafi – since May.
The militants seized the Mediterranean coastal city last year, making it their most important base outside Syria and Iraq. But they 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 air strikes on Monday – which were authorised by U.S. President Barack Obama – hit an Islamic State tank and two vehicles that posed a threat to forces aligned with Libya’s GNA, Cook said.