ISTANBUL -- There was more heavy fighting Monday along the Turkey-Syria border, but two U.S. allies seem more intent on fighting each other than ISIS.

It began with Turkey launching an offensive against ISIS last week, sending tanks and Syrian rebel fighters over its border and into the town of Jarablus.

Turkey is a close U.S. ally, and the operation was backed up by Turkish and American airstrikes.

But after an easy victory against ISIS, the Syrian rebels headed further south to take on Kurdish fighters.

Turkey views the Kurdish group as terrorists. But the Kurdish fighters are backed by the U.S. They work together with American special forces, and we’ve seen them up close stripping away territory from ISIS.

So now two U.S.-backed groups, who are both supposed to be fighting Islamic extremists, are instead clashing with each other. One video appears to show members of the Kurdish group​ captured and humiliated by rebel fighters.

A video appears to show Kurdish fighters captured by Syrian rebels. CBS News

America’s special envoy for the fight against ISIS, Brett McGurk, tweeted that the clashes are “unacceptable” and called on all sides to “stand down.”

DOD: We want to make clear that we find these clashes -- in areas where #ISIL is not located -- unacceptable and a source of deep concern. — Brett McGurk (@brett_mcgurk) August 29, 2016

The U.S. says the Kurdish fighters to withdraw from the contested area, but this isn’t the first time U.S.-backed groups have clashed.

The more they fight each other, the longer it will take them to combat ISIS​.