The U.S.-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 missile strikes against Syrian government targets in retaliation for a chemical attack on Syrian civilians.

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The stand-off assault on Syrian military facilities, the first of its kind 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 force the United States is backing in that fight, said that its fighters had fallen as “martyrs” during operations around Tabqa.

“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 proven to be a key partner for the United States in Syria.

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According to a U.S. official with knowledge of the accidental strike, a series of missteps contributed to the misguided strike.

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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.

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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 U.S.-led coalition had a strong track record of supporting SDF operations with air power.

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“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 allied have been battling since 2014.

The situation around Tabqa, however, is a particularly complex one, as Islamic State militants stage fierce battles in defense of positions near its defacto capital of Raqqa.

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U.S. officials say militants have used explosive vehicles, artillery and mortar attacks, and employed human shields in an attempt to slow forces advancing on Raqqa.

As part of the operation, U.S. 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,” the official said. “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.”

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It’s not clear whether the incident involved Kurdish SDF forces, members of a smaller Arab component, or both. The U.S. partnership with the SDF has caused intense friction with NATO ally Turkey, which sees the Kurdish fighters as a threat to their own security and has pushed the United States to instead back Turkish-selected units.

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The rise of Kurdish militias in Syria has also created friction with Syrian Arabs, some of whom resent the Kurds’ attempts to consolidate territory under their control. The United States is planning to employ a smaller Arab component of the SDF to spearhead the assault into Raqqa, an Arab city.

The Tabqa strike follows earlier, smaller friendly fire incidents in the Islamic State campaign.

In December 2015, nine Iraqi soldiers were accidentally killed by coalition aircraft in the city of Fallujah. A month later, an Iraqi drone killed nine Shiite militiamen fighting near Tikrit.

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U.S. officials argue the incidents are a regrettable but unavoidable outcome in a war that has included around 20,000 U.S. and coalition air strikes.

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But the incidents illustrate the difficulties inherent to the U.S. strategy in Iraq and Syria, which relies on local ground forces to do most of the fighting and, often, to provide information used in complex air operations.

Those challenges are made greater by the small scale of the U.S. presence in both countries, meaning fewer American forces on hand to verify targets first-hand. There are now roughly 500 U.S. Special Operations forces working with SDF throughout Syria and approximately 1,000 U.S. troops overall.

As part of an elaborate process for developing and approving targets for aerial attack, U.S. officials say they use drone surveillance and other methods to cross-check information provided by ground forces, who often are requesting air power in fast-moving self-defense situations.

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But doing so is challenging in a crowded battlefield, and requests from local forces have played a role in some incidents of civilian casualties. Those include a strike in the Syrian village of Tokhar in July 2016, and another, larger attack last month in the Iraqi city of Mosul.

In both instances, U.S. forces said they used intelligence means to try to verify targets after receiving requests from local forces, but later acknowledged casualties.

The Mosul strike, which local residents say killed at least 137 people, is now under U.S. military investigation.

The Pentagon has struggled in recent weeks to explain why there has been a surge of reported civilian casualties in its air campagin against the Islamic State. Officials say the principle cause is that the fight against the Islamic State is entering a new, more intense phase. But activists have questioned whether changes to the military’s approval process for strikes may have contributed.

Louisa Loveluck in Beirut contributed to this report.