Islamic State recaptures Palmyra after Syria army withdrawal
"Despite the ongoing air raids, IS (IS) retook all of Palmyra after the Syrian army withdrew south of the city," said Observatory head Rami Abdel-Rahman.
The ISIS-linked Amaq news agency also reported that IS regained "full control" of the city on Sunday.
Militants had stormed the famed citadel on Sunday afternoon after heavy Russian aerial bombardment killed scores of Islamic State group fighters overnight.
More than 60 airstrikes hit Palmyra overnight, Russia's defence ministry said, killing more than 300 IS militants and halting their offensive on the famed desert city in central Syria.
In a statement issued in Moscow, the defence 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, armoured vehicles and rocket artillery," it said, adding that the strikes killed more than 300 militants and destroyed 11 tanks and 31 vehicles.
The latest developments were also confirmed by the London-based Syrian Observatory for Human Rights who said "intense Russian raids since last night forced IS out of Palmyra, hours after the jihadists retook control of the city."
"The army brought reinforcements into Palmyra last night, and the raids are continuing on jihadist positions around the city," Abdel Rahman told AFP.
IS began an offensive last week near Palmyra, which is on UNESCO's World Heritage list.
The militants 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 militant 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.
Russia has carried out a bombing campaign in Syria in support of its ally President Bashar al-Assad since September 2015.