The director of Russia's renowned Hermitage Museum, which has an important collection of sculptures from Palmyra, has offered its expertise to help restore the ancient Syrian city retaken by President Bashar al-Assad's forces from the Islamic State group.

"Restoring Palmyra is the responsibility of all of us," Mikhail Piotrovsky told AFP, surrounded by displays of tomb stones, sculptures and coins from Palmyra at the museum in Saint Petersburg.

Following the IS campaign of destruction, "restoring Palmyra is a long-term task, and it's essential that we take our time," said Piotrovsky, estimating that up to 70 percent of the ancient historic site could have been damaged or destroyed by the jihadists.

"We will have to record where every stone was found before taking a decision on how to restore these historic monuments," he said of the painstaking work required.

The Hermitage director insisted that only an "international association" including UNESCO member countries and Syria's Directorate General of Antiquities and Museums should carry out the restoration of Palmyra.

Syria's antiquities chief Maamoun Abdulkarim on Friday told AFP journalists at Palmyra that he was appealing for "archaeologists and experts everywhere to come work with us because this site is part of the heritage of all humanity."

Among the highlights of the Hermitage's collection from Palmyra are four stone slabs, weighing a total of 15 tonnnes with inscriptions in Aramaic and Greek that show the customs tariffs in the 2nd century AD, when the city became an important crossroads for trading.

The slabs were brought to Russia by an aristocrat who was an amateur archaeologist, Prince Abamelek-Lazarev after he travelled to Palmyra in 1882.

The value of such intact treasures is now even greater after the destruction at the historic site.

The Hermitage chief noted that Russia has "plenty of experience with restoring destroyed historic monuments", notably after World War II.

He gave the example of Tsarskoye Selo, the tsars' summer palace outside the imperial city, which was almost entirely destroyed in fighting between Nazi and Soviet forces.

Despite this, the palace was entirely restored to the tiniest detail and is now a major tourist attraction.