Posted April 14, 2017 04:48:56

Photo: An investigation into the mistaken strike has begun. (United States Air Force, file photo)

A US-led air strike has mistakenly killed 18 members of a Kurdish and Arab militia backed by Washington south of the Syrian city of Tabqa, the Pentagon says.

The US-led coalition forces struck the position on Tuesday after another partner in the fight wrongly told them its was occupied by Islamic State militants, the Pentagon said, underlining the complex nature of the conflict.

"The target location was actually a forward Syrian Democratic Forces (SDF) fighting position," a statement said.

The SDF is fighting in a campaign to encircle and ultimately capture Raqqa city, Islamic State's main base of operations in Syria.

The militia has closed in on the Islamic State-held Tabqa area, a focus of heavy fighting, about 40 kilometres west of Raqqa.

The SDF said its leadership was working with the coalition to investigate the incident and prevent it from happening again.

"In the area of military operations near Tabqa and as a result of error, a painful incident took place [causing several casualties]," the SDF said in a statement.

An activist-run group, Raqqa is Being Slaughtered Silently, says three days of mourning have been declared in the town of Tal al-Abyad.

The Britain-based Syrian Observatory for Human Rights said 25 SDF fighters were killed in the last two days of battle.

The SDF meanwhile announced the launch of a fourth phase of their campaign to capture Raqqa, a Euphrates River city that is home to 300,000 people.

Reuters/AP

Topics: unrest-conflict-and-war, syrian-arab-republic, united-states