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A large team of Turkish rescuers began searching Syria's infamous Saydnaya prison near Damascus Monday, trying to check for any hidden underground cells, its director said.

Located just north of the Syrian capital, the prison has become a symbol of the rights abuses of the Assad clan, especially since the Syrian civil war erupted in 2011.

Prisoners held inside the complex, which was the site of extrajudicial executions, torture and forced disappearances, were freed early last week by the rebels who ousted Syrian regime leader Bashar al-Assad on December 8.

The search was being conducted "in line with a request made by the Syrian authorities," Okay Memis, the director of Turkey's AFAD disaster relief agency, said in remarks quoted by Turkish state news agency Anadolou.

Memis said his 120-strong team, which arrived with 43 vehicles, would use "ground radar and acoustic listening devices" to verify whether there were any as yet undiscovered locations underneath the facility, which he described as covering "a vast area".

The complex is thought to descend several levels underground, fuelling suspicion more prisoners could be held in as yet undiscovered hidden cells.

The team, which specialises in urban search and rescue operations, was aware that as a place where huge numbers of people had been killed, the area could be forensically important for any future criminal prosecutions, Memis said.

"We are aware there are also issues that might involve evidence, and will conduct our work with those sensitivities in mind," he said.

According to the Syrian Observatory for Human Rights, more than 100,000 people died in Syria's jails and detention centres from 2011.

More than 30,000 dead

The Association of Detainees and Missing Persons of Saydnaya Prison (ADMSP), believes the rumours about as yet undiscovered underground cells are unfounded.

The prison complex was thoroughly searched by Syria's White Helmets emergency workers but they wrapped up their operations on Tuesday, saying they were unable to find any more prisoners.

Rescuers have punched holes in walls to investigate rumours of secret levels housing missing prisoners, but found nothing, leaving many thousands of families disappointed - their relatives are probably dead and may never be found.

The Turkish team is coordinating its work with the Syrian authorities, Turkey's newly-opened Damascus embassy and the foreign and interior ministries in Ankara, Memis said.

ADMSP said the rebels freed more than 4,000 prisoners from Saydnaya, which Amnesty International has described as a "human slaughterhouse".

The organisation, which is based in southern Turkey, believes more than 30,000 prisoners died there as a result of execution, torture, starvation or a lack of medical care between 2011 and 2018.