Syrian government forces recaptured a Mamluk-era citadel in Palmyra from the extremist Islamic State group on Friday, Syrian state media and monitoring groups said, as the fierce battle for control of the historic town entered its third day.

Syrian and Russian warplanes struck at least 56 targets inside IS-held areas of the city and pro-government militias supported the army's advance, said the Syrian Observatory for Human Rights, a Britain-based monitoring group.

Palmyra has been controlled by the extremist group since May. The militants have destroyed some of its best known Roman-era archaeological relics.

Government forces Friday cut the road between Palmyra and another IS bastion, the town of Qaryatayn, weakening the group's hold over its two central Syrian outposts, according to the pro-government Lebanese Al-Mayadeen TV.

The IS militants had been relying on Qaryatayn, which is now encircled, to reinforce Palmyra's defense.

A video released by the IS news agency, dated Thursday and said to show Palmyra, depicted moderate to severe damage to the buildings in the modern town.

"God willing, we shall be victorious over the infidels," said a fatigued looking fighter in the video, sitting on top of a blown-up tank.

Government troops exchanged artillery and gunfire on the outskirts of the town, according to Syrian state media and opposition activists.

An unnamed military official cited by state-run news agency SANA said that troops earlier seized the so-called SyriaTel Hill near the Palmyra Citadel. Troops have also captured several hills around the town. They briefly entered the town on Thursday but were pushed back.

Despite the army's advance, large parts of the town, including the famed Roman ruins, remain under IS control.

The head of UNESCO welcomed the operation to unseat IS, vowing that as soon as security conditions allow, the international cultural agency is ready to go to the town along with Syrian antiquities teams "on a mission to evaluate damage and protect the priceless heritage of the city of Palmyra."

Irina Bokova said in a statement issued late Thursday that "for one year, Palmyra has been a symbol of the cultural cleansing plaguing the Middle East."

"The destruction of temples of Baal Shamin and Bel, the funeral towers and the Triumphal Arch are an immense loss for the Syrian people and the world," Bokova said. "The deliberate destruction of heritage is a war crime, and UNESCO will do everything in its power to document the damage so that these crimes do not go unpunished."