US and Syrian Kurdish officials have made much in the past few days on a planning offensive against the “last ISIS town” in Syria, Hajin. Forces have been advancing in the area around the small, relatively isolated town.

Kurdish forces, however, found themselves caught in a sandstorm moving on Hajin on Friday, and were quickly surrounded in the desert by ISIS fighters. Explosives and heavy fire were opened on the troops, leaving at least 20 of them killed.

US officials haven’t commented on the incident, and there is no indication US troops were present at the ISIS ambush. Still, the killings show ISIS, despite having lost materially all of its territory, remains capable of carrying out attacks, even against those fighters who are supposed to be advancing against them.

Estimates are that about 3,000 ISIS fighters are in Hajin, and some officials have speculated that there could be substantial leadership figures as well. With much made of the planned offensive, however, it is also likely that the highest ranking ISIS leaders have long since taken the warning and fled elsewhere, into the endless sea of Syrian desert that houses most of them.