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 IS fighters seized a strategically located but deserted rocket-launching site close to an air base less than 60 kilometers (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 IS 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 IS 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 IS-linked radio report said.

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

The IS 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 . The cease-fire was shattered in the northern city of Aleppo.

Nearly 300 people were killed in less than two weeks, in strikes that also targeted hospitals and civilian areas. Human Rights Watch quoted rescue workers as saying that in one airstrike on a hospital in a rebel-held area of Aleppo, 58 civilians were killed, including medical staff and many patients. On the other side, a government-area hospital was hit and at least 20 people were killed in shelling blamed on the rebels.

Last week, airstrikes hit a displaced people’s camp in the northern Idlib province along the border with Turkey and killed 28 people. The Russian and Syrian government denied any role.

A partial cease-fire was restored, and has been extended twice. The latest cease-fire expires at midnight Wednesday.

In Geneva, the Commission of Inquiry on Syria, a four-person U.N. team of investigators that aims to identify possible war crimes and other violations, decried strikes on medical facilities in the Aleppo and at the Idlib camp for fleeing people.

The commission noted that the recent truce has “increasingly deteriorated” and said international humanitarian law requires combatants to distinguish between “lawful and unlawful targets.”