IS, behind a string of attacks in the West including this week's Brussels bombings, is under growing pressure from Syrian and Iraqi military offensives to retake two key bastions in its self-proclaimed "caliphate".
Pro-government Syrian forces battled IS fighters today in northern and western neighbourhoods of Palmyra, a strategically located ancient desert city, under the cover of Russian and Syrian artillery fire.
In a boost to their campaign, regime forces captured the town of Al-Amiriyah on the northern outskirts of Palmyra.
An AFP journalist on a western hilltop overlooking Palmyra saw nearby artillery operated by Russian and Syrian troops firing on IS positions in the city.
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The military source said Russia was "widely involved in the battle for Palmyra, whether in fighting directly on the ground, with their planes, or by intercepting communication" among IS fighters.
Despite a major drawdown last week, Russian warplanes have continued their bombing campaign around Palmyra, conducting more than 150 air strikes in recent days.
But with pro-government forces now engaged in street battles with the jihadists, raids have markedly decreased.
"Instead, it needs heavy artillery power -- and that's what we are noticing now whether it's from the Russians or from the Syrian army."
IS overran the Palmyra ruins and adjacent modern city in May 2015, sparking a global outcry and fears for the fate of the UNESCO world heritage site known as the "Pearl of the Desert".
The group has since blown up UNESCO-listed temples and looted relics that dated back thousands of years.
Government forces began their large-scale offensive to retake Palmyra earlier this month, but Saturday marked the most significant advance yet.
"Regime forces have seized two neighbourhoods in the city's west and northwest in a simultaneous push," he told AFP.
IS was putting up a fight, however, with 10 government forces killed overnight in a car bomb west of the city, Abdel Rahman said.
The military source on the outskirts of Palmyra told AFP that he expected "the old city and the edges of Palmyra to be mined with explosives.