U.S. warplanes mistakenly 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 U.S. 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 the United States and Russia.

A U.S. defense official, speaking on the condition of anonymity to discuss the developing situation, said 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 tank and a number of other vehicles.

The official said that before the strike was called off, U.S. forces had destroyed roughly six vehicles and the “personnel associated with them.” The official would not expound on the casualty numbers. The U.S. aircraft only used guided bombs, the official said. The official also denied some Syrian news reports that the U.S. aircraft dropped incendiary bombs on the Syrian forces. Interfax news agency in Russia reported that more than 60 Syrian troops were killed.

The official added that if the Syrian government wanted to file damage claims against the U.S.-led coalition, it had “a number of avenues to do so.”

“If we did get this wrong, which it looks we did, it’s not something we intended to do,” the official said, adding that Centcom plans to investigate how targeting personnel could have confused a Syrian military unit with Islamic State fighters.

“It appears to be an intelligence failure,” the official said.

The strike was halted when Russian officials told their U.S. counterparts that U.S. aircraft were possibly targeting Syrian military personnel and vehicles. The official said, however, that even though U.S. operations over the area had ceased in the immediate aftermath of the strike, it did not preclude U.S. aircraft from returning to the area in the coming days.

Centcom said that it had informed its Russian counterparts of the strike prior to the incident and that it is not uncommon for U.S. officials to confer with their Russian counterparts to make sure there are no midair incidents.

Syria’s state-run media, echoing statements made by Syria’s general command, said the strikes allowed the Islamic State to advance against the Syrian army and indicated a “serious and blatant aggression against the Syrian Arab Republic and its army.”

While the Pentagon would not comment on what type of aircraft carried out the strike, Syrian media said the strike was carried out by four U.S. jets — two F-16 fighters and two A-10 ground-attack aircraft — flying from the Iraqi border.

Centcom said it is reviewing the strike “and the circumstances surrounding it, to see if any lessons can be learned.”

“Syria is a complex situation with various military forces and militias in close proximity,” the command’s statement said.