The Islamic State of Iraq and the Levant (ISIL) group has blown up the infamous prison complex in the central Syrian city of Palmyra, according to a monitoring group, destroying an important symbol of government control in the ISIL-held city.

The prison was empty of people at the time of the detonation, said Rami Abdulrahman from the UK-based Syrian Observatory for Human Rights. He said the facility was "largely destroyed" after ISIL fighters placed explosives inside and around it.

The demolition of the prison, for decades one of the country's most feared detention centres, came 10 days after ISIL captured the city, famed for its ancient ruins.

ISIL announced the explosion in a statement on social media and supporters posted pictures of huge clouds of grey smoke above the complex in the city, also known as Tadmur.

Syrian state media did not mention the explosion.

OPINION: Palmyra caught between two histories

The prison was the site of a massacre in 1980 in which hundreds of inmates were killed.

It became notorious throughout Syria as a symbol of the brutality of the regime of former president Hafez al-Assad and his son and successor, Bashar.

Housing political prisoners until the 2011 uprising, Palmyra's jail later became overcrowded with regime deserters and draft evaders as anti-government protests morphed into a civil war.

It was not immediately clear when the prisoners had left their cells. Activists have said inmates were relocated days before ISIL seized the city.

Pictures posted by ISIL on social media this week had purported to show empty cells, a collective detention room and a management office inside the prison.

A 2001 Amnesty International report, based on the accounts of former inmates, described the prison as "designed to inflict the maximum suffering, humiliation and fear on prisoners".