U.S. planes bombed Islamic State targets in Libya on Monday, responding to the U.N.-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. 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 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 U.S. air strikes in Libya were on an Islamic State training camp in the western city of Sabratha in February.
Seraj said the Presidential Council of his Government of National Accord (GNA), had decided to "activate" its participation in the international coalition against Islamic State and "request the United States to carry out targeted air strikes on Daesh (Islamic State)."
"I want to assure you that these operations are limited to a specific timetable and do not exceed Sirte and its suburbs," he said, adding that international support on the ground would be limited to technical and logistical help.
U.S. President Barack Obama authorised the air strikes, the White House said.
"GNA-aligned forces have had success in recapturing territory from ISIL (Islamic State) thus far around Sirte, and additional U.S. strikes will continue to target ISIL in Sirte in order to enable the GNA to make a decisive, strategic advance," said Cook, the Pentagon spokesman.
He said strikes on Monday targeted a specific tank location and two Islamic State vehicles that posed a threat to GNA forces.
U.S. and Libyan officials estimate that several hundred Islamic State fighters remain in Sirte.
Brigades mainly composed of militia from the western city of Misrata advanced on Sirte in May, but their progress was slowed by snipers, mines and booby-traps.
Those forces have complained that assistance from the government in Tripoli and external powers was slow to materialise. At least 350 of their fighters have been killed and more than 1,500 wounded in the campaign.
Libyan fighter jets have frequently bombed Sirte, but they lack the weapons and technology to make precision strikes.
Islamic State took advantage of political chaos and a security vacuum to start expanding into Libya in 2014. It gained control over about 250 km (155 miles) of sparsely populated coastline either side of Sirte, though it has struggled to win support or retain territory elsewhere in the country.
The GNA was the result of a U.N.-mediated deal signed in December to end a conflict between two rival governments and the armed groups that supported them, but it is having difficuly imposing its authority and winning backing from factions in the east.
Western powers have offered to support the GNA in its efforts to tackle Islamic State, stem the flow of migrants across the Mediterranean, and revive Libya's oil production.
But foreign intervention is politically sensitive, and the GNA has hesitated to make formal requests for help.
Small teams of Western special forces have been on the ground in eastern and western Libya for months. Last month France said three of its soldiers had been killed south of the eastern city of Benghazi, where they had been conducting intelligence operations.