The U.S. accused Russia on Thursday of continuing to allow Islamic State fighters to move freely through parts of Syria controlled by the regime of Bashar Assad, the Syrian president who is backed by Moscow.

U.S. Central Command said despite Russia turning a blind eye toward the remaining ISIS fighters in Syria, U.S.-backed “partner forces” were able to intercept an ISIS convoy near the border with Iraq, killing at least 20 militants.

“The [ISIS] terrorists freely moved through an area controlled by pro-Syrian regime forces,” the statement said. “This is the second time in less than a month that convoys of [ISIS] terrorists, fleeing the middle Euphrates River valley, transited through regime-controlled territory.”

The U.S. says the Maghawir al-Thawra, a vetted Syrian opposition force, detected the enemy convoy during the predawn hours on Wednesday and “quickly and professionally conducted an operation to prevent their further incursion.” The force received help from the U.S.-led coalition.

More than 20 ISIS fighters were killed, and an unspecified number of others were captured.

Last week, Russian President Vladimir Putin visited Russian military bases in Syria, declaring that ISIS has been defeated, and announced that Russia would begin a significant drawdown of its military forces.

The U.S. estimates that more than 2,000 fighters are still operating in Syria, although the group has lost more than 98 percent of the land it once controlled. The latest engagement is evidence that ISIS still poses a threat to the people of Syria, the U.S. says.

"While the coalition and its partner forces in Syria have weakened [ISIS], increasingly desperate terrorists continue to present a threat to the civilian population," said Brig. Gen. Jonathan Braga, the director of operations for the U.S. coalition.