The attacks mark the beginning of an ongoing U.S. air campaign in Libya, a Pentagon spokesman said.
The U.S. airstrikes came at the request of the Libyan Government of National Accord, or GNA, to support forces trying to quash ISIS in its primary stronghold in Libya, Pentagon spokesman Peter Cook said. The GNA is Libya's U.N.-backed unity government.
"GNA-aligned forces have had success in recapturing territory from ISIL 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," Cook said, using another acronym for ISIS.
"The U.S. stands with the international community in supporting the GNA as it strives to restore stability and security to Libya. These actions and those we have taken previously will help deny ISIL a safe haven in Libya from which it could attack the United States and our allies."
The White House said President Barack Obama authorized the airstrikes at the recommendation of Defense Secretary Ash Carter.
"The President's been clear that he will deny any safe haven for groups like ISIL or any group that tries to do us harm," White House spokesman Eric Schultz said. "The strikes you've seen are consistent with that approach."
It was not immediately clear how many ISIS militants or structures may have been hit.
ISIS' reach in Libya
Aside from Iraq and Syria, ISIS' presence in Libya "is probably the most developed and the most dangerous," CIA Director John Brennan
recently told Congress.
And Libya's proximity to Europe could compound the problem.
"We assess that (ISIS) is trying to increase its influence in Africa and to plot attacks in the region and in Europe," Brennan said.
Residents describe horror by ISIS
The terror group has devastated residents in Sirte, according to a Human Rights Watch report
ISIS has diverted food, medicine, fuel and cash to its members and seized homes from residents who fled, HRW said.
"There are no vegetables or meat. Most shops are closed," one resident told Human Rights Watch. "Meanwhile the Daesh (ISIS) is living in our houses and having barbecues."
ISIS has also been accused of looting and destroying homes of those it thinks are enemies.
More than two-thirds of Sirte's 80,000 residents have fled, according to the May Human Rights Watch report.
"We need help," said the mayor of nearby Misrata, Mohamed Eshtewi. "We have no more food or housing to host the people fleeing the fighting."