U.S. warplanes mistakingly struck Syrian military positions around the city of Deir al-Zour, the Pentagon said Saturday.

The strike was in a location where U.S. aircraft had targeted Islamic State militants before, according to a statement released by the United States Central Command, and prior to the errant strike, U.S. forces had been “tracking” the militants in the area. The strike occurred during a cease-fire recently brokered by United States and Russia.

A U.S. defense official, speaking on the condition of anonymity to discuss the developing situation, said that U.S. forces had been following what they thought were Islamic State militants for “some days.” The official said the militants appeared to have a number of vehicles and a tank.

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