Monday's action comes after a US strike targeted an IS training camp in a rural area near Sabratha, outside Tripoli, in February
The US military conducted air strikes in Libya on Monday, following a government request to target the Islamic State group's Libyan stronghold of Sirte, the Pentagon announced.
While the Pentagon has previously carried out air attacks on specific high-value IS targets in Libya, Monday's action marked the first known US strikes in Sirte itself.
"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 in a statement, using an acronym for the IS group.
US strikes in Sirte "will continue", Cook added without elaborating.
President Barack Obama authorised the bombings following recommendations from top pentagon officials, and the strikes were "consistent with our approach to combating ISIL by working with capable and motivated local forces", Cook added.
"The US stands with the international community in supporting the GNA as it strives to restore stability and security to Libya," he said.
Monday's action comes after a US strike targeted an IS training camp in a rural area near Sabratha, outside Tripoli, in February, likely killing an IS leader called Noureddine Chouchane and dozens of other jihadists.
In November, a US strike killed Abu Nabil, another IS leader who was also known as Wissam Najm Abd Zayd al-Zubaydi.
A US senior administration official said US action would be limited to strikes and information sharing to support these, and that
American troops would not take part in any ground operations to support the GNA.
The official said precision strikes would target key IS military infrastructure such as tanks, high-caliber weapons and command and control nodes. Libya spiraled into chaos after longtime dictator Moamer Kadhafi was ousted and killed in October 2011, with two governments vying for power and armed groups battling to control vast energy resources.
Exploiting Libya's power vacuum, IS jihadists established a foothold in Libya, especially in Sirte. The Pentagon estimates fewer than 1,000 IS fighters are in Sirte.