Islamic State forces cut in half the last Syrian government enclave in an oil-rich eastern province, pro-government media and opposition activists said Tuesday, putting new pressure on the regime after the terror group suffered setbacks elsewhere.

The group’s advances in Deir Ezzour—the capital of a province of the same name—came during a fierce Islamic State offensive. By Tuesday morning, the extremists had cut off a supply route to a Syrian military air base.

Islamic State has laid complete siege to the government-held part of the city for about two years. With the fresh advances, the group cut off an area where tens of thousands of civilians live from a Syrian air base that has been used to bring supplies and aid to the enclave. The extremists advanced despite heavy airstrikes by Syrian regime and allied Russian warplanes.

The gains against the regime come at a time when Islamic State is losing ground across its self-declared caliphate, though it did manage last month to win back control of the ancient city of Palmyra, less than a year after Syrian forces recaptured it.

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Deir Ezzour province and neighboring Raqqa province form Islamic State’s most significant territorial foothold in Syria today. In Iraq, the group is being slowly squeezed out of its last major stronghold there, the city of Mosul.

The division of the government-held enclave in Deir Ezzour threatens to worsen the humanitarian situation of some 200,000 civilians trapped in the areas that have been separated from the air base, according to the U.K.-based Syrian Observatory for Human Rights, an opposition monitoring group. The base is vital for supplying the Syrian military there and for aid to residents of the area.

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