IS "was able to cut the supply road between Homs and Palmyra near the Tayfur military airport after an attack launched from the east of Homs", the Syrian Observatory for Human Rights said yesterday.
Both cities are controlled by the regime after troops recaptured Palmyra - a UNESCO world heritage site - from the jihadist group in late March, backed by Russian air strikes.
Cutting the road "came as part of IS's biggest assault since the army recaptured Palmyra with Russian support" on March 27, Observatory chief Rami Abdel Rahman said.
"Daesh surrounds Palmyra from all angles except in the southwest", Abdel Rahman said, adding the closest IS was to the city was 10 kilometres.
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IS and the regime both control parts of the area between Homs and Palmyra.
The road IS cut is "the main road between Homs and Palmyra but not the only one as there are other side roads" between both cities, Abdel Rahman said.
Syria's civil war has killed more than 270,000 people and displaced millions since it started with anti-regime protests in 2011.