"The first American air strikes on precise positions of the Daesh (IS) organisation were carried out today, causing heavy losses... In Sirte," prime minister Fayez al-Sarraj said in a televised speech.
In Washington, the Pentagon said the raids were launched in response to a request from the unity government.
"At the request of the Libyan Government of National Accord, the United States military conducted precision air strikes against ISIL targets in Sirte, Libya, to support GNA-affiliated forces seeking to defeat ISIL in its primary stronghold in Libya," Pentagon press secretary Peter Cook said, using another name for IS.
President Barack Obama authorised the bombings following recommendations from top Pentagon officials, and the strikes are "consistent with our approach to combating ISIL by working with capable and motivated local forces", Cook added.
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"The US stands with the international community in supporting the GNA as it strives to restore stability and security to Libya," he said.
The Tripoli-based GNA launched an operation in May to retake the IS bastion of Sirte, the hometown of slain dictator Muammar Gaddafi which the jihadists have controlled since June 2015.
Sarraj stressed that the US strikes were carried out in coordination with the military command centre of pro-GNA forces, and that no foreign troops would be deployed in Libya.
"We asked for this support from the international community, notably the United States, but we want to point out that there will be no foreign presence on Libyan soil.