Syrian heritage chiefs have been involved in a major behind-the-scenes mission to save hundreds of thousands of ancient artefacts from being destroyed by ISIS fanatics.

Antiquities officials across Syria have made massive efforts to evacuate everything that could be saved from the terror group and looters.

The extent of the operation has been little known until now, but its participants described to The Associated Press a huge operation - at least 29 of Syria's 34 museums largely emptied out and more than 300,000 artifacts brought to the capital.

The pieces are now hidden in secret locations known only to the few specialists who handled them, said Maamoun Abdulkarim, who as head of the Directorate General of Antiquities and Museums in Damascus oversaw the operation. 'Other than that, no one knows where these antiquities are - not a politician, not any other Syrian.'

Syrian heritage chiefs have been involved in a major behind-the-scenes mission to save hundreds of thousands of ancient artefacts from being destroyed by ISIS fanatics. An expert is pictured hiding antiques at the National Museum in Damascus

There's much that couldn't be saved. The damage is most symbolised by Palmyra, the jewel of Syrian archaeology, a preserved Roman-era city.

ISIS militants captured it last year and proceeded to blow up at least two of its most stunning temples.

Over the weekend, Syrian government forces recaptured Palmyra from the militants and discovered they had trashed the city museum, smashing statues and looting relics — though fortunately about 400 pieces had been hidden away by antiquities officials before the ISIS takeover.

Wherever ISIS overran territory in Syria and Iraq, the jihadis relentlessly blew up, bulldozed or otherwise tore down monuments they consider pagan affronts.

They and other traffickers have taken advantage of the chaos from the five-year-old civil war to loot sites and sell off artifacts.

Even in the museums that were evacuated, some items were too large to move — giant statues or ancient gates and murals — and fell into IS hands, their fate unknown.

Jihad Abu Kahrlah, an archeologist at Syria's National Museum, works on artifacts delivered from the Daraa Museum to Damascus

These items were among tens of thousands of ancient relics and masterpieces rescued from the Deir el-Zour Museum in eastern Syria and brought to safety in Damascus

Emptied: Items have been removed from display cases and marks on the walls show where artifacts once hung in the National Museum in in Damascus, Syria. Authorities put the museum's contents into storage, part of a massive project by Syrian antiquities officials to save the country's heritage from ISIS thugs

But the 2,500 archaeologists, specialists, curators and engineers with Syria's antiquities department, including some who defected to join the opposition, have often risked death to protect what they can.

Guards at archaeological digs and other sites in areas now under ISIS control secretly keep tabs on the ruins and feed Abdulkarim photo updates on WhatsApp.

Several of them have been killed. Khaled al-Asaad, Palmyra's retired antiquities chief, was beheaded by the extremists in August after spiriting away artifacts from the city's museum.

Ziad al-Nouiji, who took over from al-Abdullah as head of antiquities in Deir el-Zour, brought a second load of relics to Damascus last June. But otherwise he has remained in the government-held part of Deir el-Zour city.

He knows the danger: ISIS militants besieging the area are hunting for him, posting his name on their Facebook pages as a wanted man. He relocated his family abroad but is staying put. 'This is my duty, my country's right. If we all left the country and our duties, who would be left?' he asked.

The antiquities authorities didn't take any chances, even clearing museums in government-controlled areas

The antiquities authorities didn't take any chances, even clearing museums in government-controlled areas. At the National Museum in Damascus, the halls and galleries have been empty since the artifacts were hidden away in 2013 for fear rebel shelling could hit the building. In the pottery room, dust rings mark where the pieces once stood and only the labels remain.

In 2014, with EU funding, the U.N. cultural agency UNESCO began training Syrian staff in storing artifacts and helped establish a nationwide system to document their inventory.

In the rebel-held northwestern city of Maarat al-Numan, archaeologists affiliated with the opposition protected the city's museum, which houses Byzantine mosaics. There the danger was from government airstrikes, so they erected a sandbag barrier with financial and logistical support from former antiquities directorate chief Amr al-Azm, who sided with the opposition. Last June, just after the sandbagging was complete, a government barrel bomb damaged mosaics in the outside courtyard, he said.