Jihadists have reduced several temples, columns and other treasures to heaps of stone in Syria's ancient city of Palmyra, which archaeologists fear will never be fully restored to its former glory.

On the rocks at the entrance to the 2,000-year-old Temple of Bel, jihadists have written in black: "The Islamic State. No entry for civilians or brothers (fighters)."

While the temple's outer walls, main entrance and courtyard have survived, the main cella or prayer chamber has been destroyed, according to AFP journalists who visited the world heritage site.

Ochre and beige-coloured blocks of stone that once formed the cella walls, rooftop and eight 16-metre (52-foot) tall fluted columns now lie on the ground.

Syria's antiquities director Maamoun Abdulkarim says he is hopeful that part of the temple can be restored now that the jihadists have fled.

"Of course the Temple of Bel will never be the same. According to our experts, we will definitely be able to restore a third of the destroyed cella, or maybe even more if we carry out additional studies with UNESCO's help," Abdulkarim said.

"It will take five years of field work."

The city was recaptured on Sunday by Syrian and Russian troops who drove out IS jihadists who had occupied it for 10 months.

In Palmyra's stunning Roman theatre, jihadists have written their names on one wall while another is riddled with bullet marks.

It was at this second-century structure that the children of IS fighters were made to kill army soldiers in public executions.

Where the cella of the shrine of Baal Shamin once stood, only four columns now remain.

And the remains of the Arch of Triumph, dating back to the era of Roman Emperor Severus in the third century, lie on the ground, leaving only the two columns that once sustained the central crown still standing.

"It won't be complicated to restore it because all the building blocks are there and the arch had already been put back up in the 1930s," Abdulkarim said.

"I invite archaeologists and experts everywhere to come work with us because this site is part of the heritage of all humanity," he said.

'Savagery'

At the National Museum, the jihadists committed some of their worst atrocities against Palmyra's heritage.

They threw several of the city's famed busts of large-eyed, ornately dressed women to the ground. They mutilated portraits. They erased the painted faces of dinner guests portrayed in ornate frescoes of funerary banquets.

"Experts believe that 30 percent of the old city of Palmyra has been destroyed," said provincial governor Talal Barazi, who came to inspect the damage.

"I have seen proof of IS's obscurantism. The damage they caused to the antiquities bears witness to their savagery," he said.

"I am happy because the museum's finest pieces were evacuated before they arrived," Barazi said, referring to 400 pieces of inestimable value that were transferred to government-held Damascus before IS took control of Palmyra.

'Eaten by dogs'

Hotels close to the museum, once buzzing with tourists, are vacant. A nearby church was turned into a recruitment centre by IS jihadists. 

The regime's Palmyra jail, which IS blew up soon after seizing the city, had been notorious for hellish torture of detainees, including political prisoners.

But IS created several makeshift prisons of its own across the city, including in a government courthouse. 

In the basement of that IS jail, a door is marked "Interrogation centre".

Behind it, a large, bare room is filled with mattresses that prisoners slept on. On the walls, detainees' names have been scratched, along with messages to loved ones.

One man wrote the name "Farah", which is Arabic for Joy, inside a big heart.

"I spent 14 days in this cell. My interrogators were Saudis, Iraqis and Tunisians. Every day, they questioned me with a sword held up to my throat," said Abu Mahmud, a former city council employee who became a pro-regime militia fighter after he was freed.

"I was lucky to be released, but I have civil servant friends who were executed, and whose bodies were thrown in the desert and eaten by dogs."

The streets are marked by potholes from the blasts of mines planted by the jihadists during their retreat.

"Palmyra had a narrow escape. IS had planted 4,500 improvised mines across most of the city, linked to each other by mobile phone to the telephone network," Abu Mahmud said.

A regime loyalist went undercover and "killed the man who was tasked with triggering the explosions", he said. Barazi confirmed the report.

Now, an explosion can be heard every half an hour.

"It's the Syrian army's engineering unit, waiting for Russian deminers to arrive in the coming days," Barazi said.