A US drone struck and killed at least 18 members of an allied Syrian force this week, the Pentagon said, in the worst friendly-fire incident of the war against the Islamic State. The strike Tuesday south of Tabqa, a strategic town in northern Syria, deepens questions about targeting methods used in the ongoing American air campaign over Iraq and Syria, which activists allege has resulted in a surge in civilian deaths this year. The US-led coalition said this week’s incident occurred after Syrian forces erroneously identified another allied unit as a group of Islamic State fighters. The incident comes a week after the Trump administration, promising a tough stance on a range of foreign-policy issues, launched a barrage of ship-borne cruise missiles against a Syrian government air base in retaliation for a deadly chemical attack on Syrian civilians. The assault using standoff weapons against a Syrian military facility, the first direct US attack on a Syrian government target since the Syrian conflict began in 2011, appeared to be a momentary deviation from the campaign the United States and its Syrian allies are waging to defeat the Islamic State. The Syrian Democratic Forces (SDF), an opposition grouping the United States is backing in that fight, said that its fighters had fallen as “martyrs” during operations around Tabqa. Syria’s chemical attack ‘100 per cent fabrication’, says President Assad “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,” the SDF General Command said in a statement. The SDF is a Kurdish-dominated force that has proved to be a key partner for the United States in Syria. According to a US official with knowledge of the accidental strike, a series of missteps contributed to the errant strike. First, an SDF unit operating close to Islamic State lines incorrectly reported its own location to the US-led coalition, the official said. Typically, friendly forces share their locations with the United States to keep themselves safe from foreign air power. Syria’s Assad, hated by the West but defended by Russia Then, a separate SDF unit, which spotted the first unit from afar, mistakenly reported its fellow SDF fighters to be Islamic State elements, requesting an airstrike 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. In an email, the official said the US-led coalition had a strong track record of supporting SDF operations with air power. “The Coalition is in close contact with our [Syrian] partners who have expressed a strong desire to remain focused on the fight against ISIS despite this tragic incident,” the coalition said in a statement. ISIS is another name for the Islamic State, the militant group the United States and its allies have been battling since 2014. The situation around Tabqa, however, is particularly complex, as Islamic State militants stage fierce battles in defence of positions near their de facto capital, Raqqa. US officials say militants have used vehicles fitted with explosives, artillery and mortar attacks, and have employed human shields in their efforts to slow forces advancing on Raqqa. As part of the operation, US Special Operations forces and SDF troops landed behind Islamic State lines in the Tabqa area last month, aiming to capture the town as well as its strategic dam and airfield. “This was a very dynamic situation,” said the US official who commented on the accidental airstrike. “The ground units and coalition forces involved in this strike are well experienced and communicate often. Unfortunately, this dynamic situation resulted in loss of detailed location understanding.” It is not clear whether the incident involved Kurdish SDF forces, members of a smaller Arab component or both. The US partnership with the SDF has caused intense friction with Nato ally Turkey, which sees the Kurdish fighters as a threat to Turkish security and has pushed the United States to instead back Turkish-selected units. In December 2015, nine Iraqi soldiers were killed by coalition aircraft in the city of Fallujah. A month later, an Iraqi drone killed nine government-allied Shiite militiamen fighting near Tikrit. US officials argue that the incidents are a regrettable but unavoidable outcome in a war that has included about 20,000 U.S. and coalition airstrikes.