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.
The Observatory also reported the downing of the aircraft but said the fate of its crew remains unclear.
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Today'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 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.