Security experts estimate there are as many as 3,000 fighters loyal to the Islamic State in Libya. The country has become one of the primary locations to train with the group outside of Syria and Iraq. Volunteers from Tunisia, Algeria, Egypt, Saudi Arabia and other countries have flocked here to fight with the extremists and other jihadist organizations. The Islamic State also has succeeded in pulling away members of other Libyan extremist groups.

In the latest signs of their growing strength, Islamic State fighters last month seized the airport and an adjacent air base in Sirte, where they have controlled most government institutions since February. The militants also took over the nearby headquarters of a mammoth network of pipes that pump fresh water to Libyan cities.

Then, on May 31, the militants dispatched a Tunisian suicide bomber to Misurata, 170 miles west of Sirte. He rammed his vehicle into a major security checkpoint, killing five.

Five days later, Islamic State fighters captured the town of Harawah, 46 miles east of Sirte.