U.S. military jets launched a new round of airstrikes against the Islamic State terror group in Libya Monday, Pentagon officials aounced, saying the United Nations-backed Libyan govement made the request.

The strikes were authorized by President Obama, the Pentagon said.

"[The strikes] are consistent with our approach to combating ISIS by working with capable and motivated local forces," Pentagon Press Secretary Peter Cook said in a statement. "These actions and those we have taken previously will help deny ISIS a safe haven in Libya from which it could attack the United States and our allies."

Fayez Serraj, the head of the U.N.-brokered presidency council, said in a televised statement that American warplanes attacked the ISIS bastion of Sirte, adding that no U.S. ground forces were deployed.

"The presidency council, as the general army commander, has made a request for direct U.S. support to carry out specific airstrikes," he said. "The first strikes started today in positions in Sirte, causing major casualties."

The strikes mark the start of a more intense American role in the fight against ISIS in Libya, as the U.S. steps in to assist the fragile govement there. Sirte represents the terror group's only remaining stronghold along Libya's northe coast.

Libya slid into chaos after the 2011 ouster of Moammar Qaddafi and has in the past years become bitterly divided between rival factions and competing govements.

U.N. Secretary-General Ban Ki-moon said on July 19 that ISIS fighters in Libya were facing the "distinct possibility" of defeat in their last stronghold and were likely to scatter elsewhere in the North African country and the region.

The U.N. chief said in a report to the U.N. Security Council that member states' estimates of the number of ISIS fighters ranged between 2,000 and 7,000 from Libya, Tunisia, Algeria, Egypt, Mali, Morocco and Mauritania.

Ban said one member state recently reported between 3,000 and 4,000 ISIS fighters in Sirte.

But he said as a result of the recent offensive against ISIS, by forces aligned with the U.N.-brokered govement and others, "the current number of those in Sirte is now likely well under 1,000," with large numbers of those who have recently fled the city likely relocating and regrouping "in smaller and geographically dispersed cells throughout Libya and in neighboring countries."

According to one uamed U.N. member state, Ban said dozens of Tunisians fighting for ISIS already retued home "with the intent to conduct attacks."

The secretary-general said the current political divide in Libya continued to make the country attractive for foreign fighters who are actively recruited by ISIS and Al Qaeda-linked groups to boost their military capabilities.

Since 2014, Libya has been split between rival govements and parliaments based in the weste and easte regions, each backed by different militias and tribes. A U.N.-brokered deal in December to create a unity govement has reached a deadlock, due to political jockeying and the new govement's inability to put together an action plan to provide basic services.

Ban said ISIS propaganda in March 2015 and again in April 2016 called for supporters to travel to Libya instead of Iraq or Syria. He said the largest group of foreign fighters currently operating in Libya are individuals linked to ISIS, and they dominate the extremist group's top leadership.

Foreign fighters have also joined Al Qaeda in the Islamic Maghreb which periodically infiltrates Libya and uses the country as a rest stop and for training and buying arms, he said.

Another prominent group of foreign fighters in Libya are members of Ansar Al-Sharia in Tunisia, another Al Qaeda affiliate, which continues to train its forces in Libya near the Tunisian border, the secretary-general said.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.