US planes bombed have bombed targets of the so-called Islamic State in Libya, responding to the UN-backed government's request to help push the militants from their former stronghold of Sirte.
US officials described the strikes as the start of a sustained campaign against the extremist group in the city.
Pentagon spokesman Peter Cook said the strikes did not have "an end point at this particular moment in time".
Forces allied with Libyan Prime Minster Fayez Seraj have been battling IS 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.
Mr Seraj said the Presidential Council of his Government of National Accord had decided to "activate" its participation in the international coalition against IS and "request the United States to carry out targeted airstrikes on Daesh (Islamic State)."
The air strikes - which were authorised by President Barack Obama - hit an IS tank and two vehicles that posed a threat to forces aligned with Libya's GNA, Mr Cook said.
In the future, each individual strike will be coordinated with the GNA and needs the approval of the commander of US forces in Africa, he added.
This was the third US air strike against IS militants in Libya. But US officials said this one marked the start of a sustained air campaign rather than another isolated strike.
The last acknowledged US air strikes in Libya were on an IS training camp in the western city of Sabratha in February.
Although it does not include the use of ground troops beyond small special forces squads rotating in and out of Libya and drones collecting intelligence, the air campaign opens a new front in the war against IS and what US officials consider its most dangerous component outside Syria and Iraq.
Mr Obama authorised the strikes after a recommendation by US Secretary of Defense Ash Carter.
Washington took part in airstrikes in 2011 to enforce a no-fly zone in Libya which helped topple Gaddafi.
The country has struggled since then and Mr Obama said in an interview with The Atlantic magazine in April that the intervention "didn't work".
