The UK Daily Mail reports that an April 11 airstrike in Syria by U.S. coalition planes accidentally hit a Syrian Democratic Forces unit, killing 18 “friendly fighters.”

“The strike was requested by the partnered forces, who had identified the target location as an ISIS fighting position,” said a statement from the Pentagon. “The target location was actually a forward Syrian Democratic Forces fighting position. The coalition’s deepest condolences go out to the members of the SDF and their families. The coalition is assessing the cause of the incident and will implement appropriate safeguards to prevent similar incidents in the future.”

“The general leadership of SDF in coordination with [the] international coalition will investigate the reasons behind the accident in order to prevent it happening again,” said the SDF general command in a statement.

The SDF called the incident a “painful accident” that resulted from a “mistake.”

The Daily Mail notes that the Kurdish-led SDF has been fighting ISIS for control of the strategic town of Tabqua, located near the Euphrates River, since March. Coalition air power has been called in by the SDF on previous occasions during this battle.

The Washington Post notes that human rights activists have complained about rising civilian casualties in Syria, in part because air support is being called into fierce battles near civilian populations. Greatly exacerbating this problem is the Islamic State’s penchant for using captive civilians as human shields.

The Post offers a more detailed breakdown of the accidental bombing, provided by a U.S. official:

First, an SDF unit operating close to Islamic State lines incorrectly reported its own location to the U.S.-led coalition, the official said. Typically, friendly forces share their locations with the United States in order to keep them safe from foreign air power.

Then, a separate SDF unit, which spotted the first unit from afar, mistakenly reported its fellow SDF fighters to be Islamic State, requesting an air strike on their location. Armed with what American officials believed were coordinates for a legitimate target, the drone then conducted the attack, with deadly results.

The strike occurred at night and the SDF units did not have night-vision gear, the official said.

The SDF includes Assyrians, Turkmen, Armenians, and Arabs in addition to the Kurds. It is not currently known which of these ethnic groups comprised the unit struck by the accidental airstrike.