An airstrike on a rebel training camp in northwestern Syria on Monday killed more than 50 Turkish-backed fighters and wounded nearly as many, a Syrian opposition spokesman and a war-monitoring group said.

The airstrike in the northwestern part of Idlib province, the last rebel stronghold in Syria, targeted a military training camp for Failaq al-Sham, one of the largest Turkish-backed opposition groups in Syria, said Youssef Hammoud, a spokesman for the groups.

The Britain-based Syrian Observatory for Human Rights, which monitors the war in Syria, gave a higher toll, saying 78 fighters were killed and nearly 90 wounded. Rescue missions were still underway, the organization said, adding that it suspected the airstrike was carried out by Russia, which is a close ally of Syrian President Bashar Assad in the country’s civil war.

Leaders of the camp were among those killed in the airstrike in Jabal al-Dweila, Hammoud said. The camp is close to Syria’s border with Turkey.

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Syrian rebel groups vowed to retaliate.

“The factions of the National Front for Liberation … will respond to these violations,” said Naji al-Mustafa, another spokesman for the Turkish-backed fighters, who threatened to target government and Russian posts. He called the airstrike a “crime” by Russia.

World & Nation

Syrian advance sends hundreds of thousands fleeing in Idlib

Airstrike in northwestern Syria kills dozens of rebel fighters

World & Nation

Syrian advance sends hundreds of thousands fleeing in Idlib

As Syrian and Russian warplanes pound hospitals and schools, civilians pack up and stream toward the Turkish border, in temperatures hovering around freezing.

Turkey and Russia had brokered a truce in Idlib earlier this year to halt a government offensive that displaced hundreds of thousands of residents. But the truce remained shaky.

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Turkey has long supported Syrian rebel forces in Syria. Russia has negotiated with Turkey to deploy observation teams in the rebel enclave to monitor the truce.

Last week, Turkish troops evacuated one of their largest military bases in the area, which was surrounded by Syrian government troops for months. Syrian opposition fighters said it was part of Turkey’s redeployment of its forces in the shrinking enclave.