WASHINGTON/AMMAN (Reuters) - A Pentagon-backed rebel group and the U.S.-led coalition fighting Islamic State killed more than 20 Islamic State fighters and detained a number of militants in the southern Syrian desert near a base, the coalition said on Thursday.

In a statement, the coalition said the Maghawir al Thawra, the rebel group, and the coalition detected a convoy near at-Tanf, near the Syrian border with Jordan and Iraq, early on Wednesday and carried out an operation “to prevent their further incursion.”

“Despite the presence of Russian-backed, pro-Syrian regime forces in the area, Daesh still finds ways to move freely through regime lines and pose a threat,” Brigadier General Jonathan Braga, the director of operation for the coalition, said, using an Arabic acronym for Islamic State.

A U.S. official, speaking on the condition of anonymity, said the convoy had come within the established 55 km (34 miles) around the Tanf base, where U.S. special forces operate.

Another U.S. official said more than a dozen militants had been captured by the rebel group.

The statement added that Islamic State militants have been moving freely through areas controlled by pro-Syrian regime forces.

“This is the second time this month small groups and convoys of Daesh are moving from the eastern areas of Syria toward the southern area through areas controlled by the regime, the Russians and Shi’ite militias,” Colonel Muhanad al Talaa, commander of Maghawir al Thawra told Reuters.

“There are Daesh convoys that are moving and that the regime and the Russians are not seeing them, this is a matter that is strange,” Talaa said.