The Islamic State (ISIS) jihadist group on Sunday recaptured the Syrian ancient city of Palmyra, after Syrian armed forces pulled out of the desert city, the Syrian Observatory for Human Rights said, according to AFP.

"Despite the ongoing air raids, ISIS retook all of Palmyra after the Syrian army withdrew south of the city," said Observatory head Rami Abdel Rahman.

The jihadists made a lightning-fast advance across the city after overrunning a northern neighborhood and capturing the famed citadel to Palmyra's west.

The ISIS-linked Amaq news agency also reported that the jihadist group regained "full control" of the city on Sunday after taking the citadel, which overlooks Palmyra from a strategic hilltop.

ISIS launched an offensive last week near Palmyra, a renowned UNESCO World Heritage site.

It seized oil and gas fields before making a major push into the desert city on Saturday, sparking new worries for Palmyra's remaining ancient treasures.

But a fierce Russian bombing campaign killed scores of ISIS fighters and forced others to withdraw at dawn on Sunday before the group recaptured the city.

ISIS overran Palmyra, known as the "Pearl of the Desert", in May of 2015 and proceeded to blow up UNESCO-listed temples and looted relics that dated back thousands of years.

The jihadist group used Palmyra's grand amphitheater for a massacre in which child members of the group killed 25 Syrian soldiers, execution-style, in front of residents.

It also beheaded Palmyra's 82-year-old former antiquities director in August.

AFP contributed to this report.