U.S. warplanes hit the ISIS stronghold of Sirte in Libya in support of the Tripoli government’s efforts to retake the port city, the Pentagon said Monday.

The airstrikes were authorized by President Barack Obama on the recommendation of Defense Secretary Ashton Carter and Joint Chiefs Chairman Marine Gen. Joseph Dunford, Pentagon Press Secretary Peter Cook said in a statement.

The release didn’t describe the duration of the strikes, the type of aircraft used or whether they were land- or carrier-based.

The airstrikes were requested by the Libyan Government of National Accord, which is based in Tripoli and is supported by the United Nations, according to the statement. The strikes were aimed at defeating ISIS “in its primary stronghold in Libya,” it said.

Efforts to retake Sirte have bogged down in recent weeks, and the U.S. will provide additional airstrikes to oust ISIS from the city, the statement said.

“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,” the statement said, using another acronym for the Islamic State of Iraq and Syria.

“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,” it added.

ISIS established its primary north African base in Sirte last year, taking advantage of political upheaval in Libya following the ousting of ruler Moammar Gadhafi.