Syrian forces backed by Russian airstrikes drove ISIS militants from Palmyra on Sunday, state media and an opposition monitoring group said.

Government forces had been on the offensive for nearly three weeks to try to recapture the central town, which fell to ISIS in May. The recapturing of Palmyra ends the group’s reign of terror over a town whose famed 2,000-year-old ruins once attracted tens of thousands of visitors.

The advance of Syrian forces marks the latest setback suffered by ISIS, which has come under mounting pressure on several fronts in Iraq and Syria.

Syrian president Bashar Assad described the offensive as a "significant achievement."

In comments reported by Syrian state TV Saturday, he said that the overthrow of ISIS in the historic town offered "new evidence of the effectiveness of the strategy espoused by the Syrian army and its allies in the war against terrorism."

The Syrian Observatory for Human Rights confirmed ISIS lost the town, saying there were around 400 extremists killed, according to Sky News. Activists added that some fighters withdrew from Palmyra toward the town of Sukhna and other areas in the Homs province.

"That's the heaviest losses that ISIS has sustained in a single battle since its creation," Observatory director Rami Abdel Rahman told Sky News. “It is a symbolic defeat for ISIS comparable with that in Kobani.”

On Saturday, Russia's defense minister said Russian jets carried out 40 air sorties near Palmyra, hitting 158 targets and killing over 100 militants.

Russia’s airstrikes have given government forces a boost on a number of fronts in recent months.

Syrian state TV interrupted its normal programs to air a documentary about the town and its archaeological sites. The network earlier aired national songs as well as videos released by ISIS showing its fighters committing atrocities in Syria and Iraq.

"It's 10 in the morning Palmyra time. Our morning is victorious," a TV announcer said.

Later a TV reporter spoke live from inside Palmyra, showing troops in the center of the town. Some of the nearby buildings had been reduced to rubble.

Palmyra, affectionately known as the "Bride of the Desert," used to attract tens of thousands of tourists every year. ISIS drove out government forces in a matter of days and later demolished some of the best-known monuments in the UNESCO world heritage site. The extremists believe ancient ruins promote idolatry.

ISIS had used Palmyra’s ancient amphitheater as a venue for public executions, including the beheading of the city’s former antiquities chief, Sky News reported.

The militants also demolished the town's infamous Tadmur prison, where thousands of Syrian government opponents were reported to have been tortured.

The Associated Press contributed to this report.

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