The United States ramped up its air campaign against the self-proclaimed Islamic State in Libya over the weekend, increasingly targeting jihadist snipers positioned in buildings around the city of Sirte, a U.S. defense official said.

Since the U.S. military opened a new front against ISIS by launching strikes in Libya, the terror group has adjusted its tactics, U.S. officials said. ISIS still depends on snipers to maintain its control of the Libyan city, but the group is no longer as mobile because of U.S. airpower.

“ISIS knows we are in the air, and they are adjusting,” a U.S. defense official told The Daily Beast.

The ISIS fighters appear to be restricting their movement during the day in an attempt to avoid the air strikes.

A U.S. official said that change may have contributed to a more aggressive air campaign over the weekend, which is being directed by the Libyan government. The U.S. military conducted nine strikes in the Libyan city Sirte over the weekend. That marked an increase in the rate of the strikes, since the U.S. conducted only 11 strikes over the prior five-day period. The total number of U.S. strikes in Libya, which began Aug. 1, is now 20.

The defense official said U.S. strikes were not pre-planned, but rather were a product of “emerging targets.”

The air strikes are meant to aid pro-government militias who are seeking to take Sirte back from ISIS control. The militiamen have stalled just outside the Libyan city’s center, an urban area covering about four miles, according to The Washington Post.

When the U.S. military campaign began, officials said they had hoped to move conservatively as they believe the campaign to retake Sirte must be seen as Libyan-led.

Less than 500 ISIS fighters remain in former Libyan leader Moammar Gadhafi’s hometown, which now is ISIS’s north African capital, the U.S. defense official said. ISIS has placed snipers, land mines, and booby traps throughout Sirte, making the city increasingly dangerous for pro-government militias.

Since May, at least 350 Libyan brigade members have been killed and more than 1,500 wounded while fighting ISIS in the Libyan city.

—Alexa Corse