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The Syrian Observatory for Human Rights, a monitoring group, described the attack as ISIL’s most significant for months. It has raised fears that Russia, by hitting rebel groups that are fighting ISIL as well as the regime, will inadvertently strengthen the hardline jihadists they say they want to crush. Many rebels say the policy is deliberate, and that Russia is pursuing a strategy Assad devised, of deliberately assisting ISIL so that the West is given a choice of supporting the regime or ISIL.

On Thursday, Ashton Carter, the U.S. defence secretary, accused Moscow of inflaming extremism.

“By taking the side of Assad they inflame the civil war — and therefore extremism — and prolong the suffering of the Syrian people,” Carter said in London. “They are going to have the effect also of turning everyone against Russia itself. This will boomerang in a very direct way on Russia.”

A Syrian regime offensive against non-ISIL rebels backed by the Russians and Syrian helicopters has foundered in the face of determined resistance.

Videos released online by rebel groups purported to show them destroying a number of regime tanks this week with TOW missiles, which are American-made but supplied by Saudi Arabia. The Saudis were reported to have approved the delivery of another 500 TOW missile systems to the rebels in response to the Russian intervention in the war. Provision of surface-to-air missiles, a demand of the Syrian opposition, has been rejected at the behest of the United States, which fears they will end up in the hands of jihadists.

The Pentagon said it was “pausing” its own program to train rebel groups to fight ISIL, and would instead focus on equipping and providing direct air support to approved groups.