The future of a partial cease-fire in Syria’s civil war plunged even deeper into doubt after an airstrike by the U.S.-led international military coalition mistakenly killed dozens of Syrian government troops on Saturday.

The attack further undermines an already delicate truce agreement that called for coordinated U.S.-Russian air campaign against ISIS and other jihadist groups following a weeklong reduction in violence which was set to be completed by September 19.

The accord negotiated by the U.S. and Russia was less than a week old when the air raid hit Syrian regime troops in Deir al-Zour, a remote city in eastern Syria where government forces and civilians are surrounded by Islamic State fighters.

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The Russian defense ministry said 62 Syrian soldiers were killed and more than 100 others wounded in the strike. The U.S. military’s Central Command said international coalition forces thought they were attacking ISIS fighters, and then halted airstrikes after they were informed by Russia that Syrian troops may have been killed.

The killings have infuriated the Russian government, which launched a large-scale campaign of airstrikes in support of its ally the Assad regime nearly a year ago. Russia called an emergency session of the United Nations Security Council in response to Saturday’s attack, which is sure to complicate talks over the fate of the already fragile ceasefire. Some Russian officials have gone so far as to insinuate that the attack was deliberate.

“It is very strange to believe that it was a coincidence,” said Vitaly Churkin, Russia’s ambassador to the United Nations, according to the TASS news agency on Sunday. “The time (of the airstrike), as well as other aspects of the situation, point to the possibility that it might have been a provocation.” The U.S. ambassador to the U.N., Samantha Power, accused Russia on Saturday of “cheap point scoring and grandstanding.”

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The international coalition attack in Deir al-Zour on Saturday also illustrates the complexities of American strategy in Syria, where the U.S. is pursuing both a military campaign against ISIS and a diplomatic effort with Russia aimed at deescalating the conflict between the Assad regime and its rebel opponents. The U.S. called on Assad to step down following the 2011 popular uprising against his regime, an initially peaceful revolt that spiraled into the current civil war which has left nearly a half million people dead. The current ceasefire deal calls on Russia to restrain the Assad regime from launching airstrikes which have devastated opposition-held areas of Syria.

“I think the ceasefire agreement is just going to collapse,” says Lina Khatib, head of the Middle East and North Africa program at Chatham House in London. “In the end, all these attempts at finding short term measures to try to lessen the extend of violence in Syria at the moment are not the result of a solid political agreement at the international level. We are only seeing half hearted, half-baked initiatives taken by the U.S. and Russia with the latest ceasefire being just the most recent example,” she added in a phone interview.

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The Russian-U.S. agreement produced relative calm in Syria last week, but the truce appeared to be fraying even before the deaths of the Syrian soldiers. Since Friday, Activists with the Syrian opposition reported renewed shelling by both regime and Russian forces elsewhere in Syria including the reported use of incendiary munitions in Homs province on Saturday. In a separate violation of the terms of the ceasefire, the Syrian regime continued to deny access to humanitarian aid convoys bound for the besieged rebel-held section of the city of Aleppo. Fighting was also reported on the outskirts of Damascus.

Australia confirmed on Sunday that its warplanes were involved in the ill-fated coalition operation on Saturday. In a statement the Australian defense department echoed the U.S. military, saying, “Australia would never intentionally target a known Syrian military unit or actively support Daesh (ISIS).” It’s unknown exactly what role the Australian aircraft played in the attack, or whether other nations’ planes were involved.

The Assad regime maintains its control of an isolated island of territory in Deir al-Zour, where an estimated 100,000 residents are surrounded by Islamic State-controlled territory. The United Nations began airdropping food into the enclave in January. The city lies on a strategic route linking ISIS territory in Syria and neighboring Iraq, including the Iraqi city of Mosul, the largest urban area controlled by the jihadists.

The Syrian military initially said that ISIS was able to gain ground following the mistaken airstrike, which reportedly took place near the Deir al-Zour airport. Syrian state later reported that government troops reclaimed the area, killing dozens of ISIS fighters in the process. Separately, ISIS’ official media arm claimed that its forces shot down a Syrian warplane over the city.

The strike was the first known attack by U.S.-led forces on the forces of the Assad the regime. The Obama administration threatened to use force against the regime in 2013 in response to a chemical weapon attack outside of Damascus that killed an estimate 1,400 people, but backed down following a Russian-backed international agreement to destroy Syria’s known chemical weapon stockpile.

The U.S.-led international military coalition, which has been waging war on ISIS in both Iraq and Syria since 2014, acknowledged that the deadly airstrike, saying that it “may have mistakenly struck a Syrian military unit and destroyed Syrian military vehicles.” The coalition also said it carried out three separate strikes in Deir al-Zour on the same day that destroyed seven of ISIS’ oil tanker trucks and “damaged eight supply routes.”

Syrian opposition activists have recently criticized the United States for indirectly assisting the Assad government by coordinating with Russia to target jihadist groups while allowing the regime to continue its deadly air strikes in rebel-controlled territory.

Some analysts suggested that the location of the strike in Deir al-Zour suggested the raid could have been a rare, even unprecedented attempt to assist regime forces battling ISIS in the area. The U.S. offers air support to Kurdish-led militias battling ISIS in northern Syria, and has sent assistance to some rebel groups opposed to both ISIS and the regime, but has not been known to coordinate with Assad’s forces in the past. “U.S. airstrikes on ISIS in such close proximity to regime positions are unusual. Arguably constitute close air support for regime,” tweeted Faysal Itani, a senior fellow at the Atlantic Council in Washington.

In July, human rights groups raised alarm over U.S.-led airstrikes that reportedly killed more than 100 civilians over a span of weeks near the Syrian city of Manbij. The coalition helped Kurdish-led militias retake the city from ISIS in August. Since 2014, at least 1,595 civilians have been killed in Syria by U.S.-led coalition airstrikes, according to the independent London-based monitoring group Airwars.

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