A Russian aerial onslaught killed scores of Islamic State group fighters in Syria's Palmyra on Sunday and forced others to withdraw hours after they had re-entered the ancient city, Moscow said.
Russia's defense ministry said its warplanes carried out more than 60 strikes overnight on Palmyra, killing more than 300 ISIS jihadists and halting their offensive on the famed desert city in central Syria.
"Intense Russian raids since last night forced ISIS out of Palmyra, hours after the jihadists retook control of the city," said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
"The army brought reinforcements into Palmyra last night, and the raids are continuing on jihadist positions around the city," Abdel Rahman told AFP.
In a statement issued in Moscow, the defense ministry said Russian warplanes conducted 64 air strikes against "positions, convoys and advancing reserves of militants" in Palmyra.
"Over the past night, Syrian government troops with active support of the Russian air force thwarted all terrorist attacks on Palmyra," it said in a statement.
"The attacking militants actively used car bombs with suicide bombers, armored vehicles and rocket artillery," it said, adding that the strikes killed more than 300 militants and destroyed 11 tanks and 31 vehicles.
Russia has carried out a bombing campaign in Syria in support of its ally President Bashar al-Assad since September 2015.
ISIS began an offensive last week near Palmyra, which is on UNESCO's World Heritage list.
The jihadists killed around 50 members of Syrian government forces after launching simultaneous attacks on several regime positions near Palmyra on Thursday, the Britain-based Observatory said at the time.
They targeted areas including near the Mahr and Shaar oil and gas fields and seized government checkpoints, silos and the village of Jazal, northwest of Palmyra.
In May last year, the Sunni Muslim extremist group seized several towns in Homs province including Palmyra, where they caused extensive damage to many of its ancient sites.
They were ousted from Palmyra in March by Syrian regime forces backed by Russia.
The recapture of Palmyra was hailed as a major victory, with Russian celebrities travelling there since March staging concerts and making public appearances.
Moscow has been under severe criticism for its air strikes on Aleppo - which it says it stopped on October 18 - where the anti-Assad opposition is currently holed up in just a fraction of the territory it once controlled.
The city's eastern districts are still being bombed by the Syrian regime which Washington has labelled "war crimes" and a UN General Assembly demanded an immediate ceasefire to stop the carnage.