The United States has carried out air strikes on Islamic State positions in Libya, following a request by the United Nations-backed government there.
The Pentagon said the strikes targeted positions in the port city of Sirte, an IS stronghold.
In a televised address, Libyan Prime Minster Fayez Seraj said the attacks caused heavy losses.
Forces allied with Mr Seraj's government 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 central hospital and the university.
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 last acknowledged US air strikes in Libya were on an Islamic State training camp in the western city of Sabratha in February.
Pentagon spokesman Peter Cook said the strikes did not have "an end point at this particular moment in time".
US President Barack Obama authorised the air strikes, the White House said.
Strikes on Monday targeted a specific tank location and two Islamic State vehicles that posed a threat to government forces.
US 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 1500 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 250km of sparsely populated coastline either side of Sirte, though it has struggled to win support or retain territory elsewhere in the country.
Libya's Government of National Accord (GNA) was the result of a UN-mediated deal signed in December to end a conflict between two rival governments and the armed groups that supported them, but it is having difficulty imposing its authority and winning backing from factions in the east.