BEIRUT — Islamic State militants advanced toward the Syrian city of Palmyra on Wednesday, threatening to besiege the world-famous ancient site only several weeks after the government recaptured it from the extremists.

Media allied with the group and other activists said Islamic State fighters seized a strategically located but deserted rocket-launching site close to an air base less than 40 miles from Palmyra. For the government forces, the capture effectively severs a highway linking Palmyra to the government-controlled T-4 air base and the provincial capital Homs, threatening government supply routes.

The Britain-based Syrian Observatory for Human Rights and other activists confirmed the reported Islamic State advance. The development comes after intense clashes with government troops near the air base, and a week after the extremist group advanced toward natural gas fields to the north.

Al-Bayan radio reported that Islamic State militants took control of the deserted site, seized two government checkpoints guarding the air base and downed a military helicopter to the north of the base. The observatory also reported the downing of the aircraft but said the fate of its crew remains unclear.

Wednesday’s capture “helps in severing the supply routes of the (Syrian) army from T-4 base to Palmyra, and tightening the siege on the city,” the Islamic State-linked radio report said.

Syrian troops, with the help of Russian airstrikes, regained control of the world-famous ancient city in March, after the Islamic State had controlled it for nearly 10 months. During their occupation, the Islamic State destroyed many of Palmyra’s relics and displaced its residents.

The Islamic State advance comes despite a partial cease-fire that excluded the extremist group and its rival al-Qaida branch, the Nusra Front, and was technically intended to focus government and allied forces efforts toward combatting the extremists.

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