Islamic State (Isis) jihadists have executed six alleged spies by burning them to death in boiling vats of tar, say reports. Multiple reports from the IS-controlled city of Mosul in northern Iraq suggest that the men were killed for assisting the US-backed Iraqi government or Kurdish Peshmerga forces in the area.

Iraq's second city is based in the northwestern Nineveh province. The extremists have carried out a number of shocking beheadings and executions in Mosul after convictions in Sharia courts and at the behest of the feared al-Hisbah religous police.

The executions were said to have taken place in front of a crowd in the central al-Shora square and some reports suggest that the jihadists executed 19 people by burning, beheading and crucifixion in a bloody three-day period in the city.

One activist told ARA News, a Kurdish news organisation: "They were accused of spying for the US-led coalition and cooperating with the Kurdish Peshmerga forces, who have been attacking IS (Daesh) headquarters in Mosul and its countryside for months.

The reports, which have not been independently confirmed by IBTimes UK, also quoted an eyewitnesses who said an IS official read a statement to the crowd before carrying out the execution, saying: "Those mercenaries have cooperated with the crusader coalition and the Peshmerga in order to destroy the Caliphate territory and kill innocent Muslims in the name of freedom and democracy. The Caliphate [Islamic State] will show no mercy to such traitors."

Mosul was overrun by IS militants in June 2014 and despite remaining under IS control, many of the the group's gains have since been clawed back by the Iraqi army backed by US-led coalition air strikes and Kurdish forces.

Mosul is the largest city in Iraq still under IS control, but is facing increasing pressure from the north, south and east by the Iraqi army earlier this year. US President Barack Obama predicted the city would fall by the end of this year.

The latest execution mimics a report from the city in May which suggested 25 alleged spies were executed after being lowered into a huge tub of nitric acid, until their 'organs dissolved'. The nature of the conflict in Syria and Iraq makes reports like these almost impossible to verify.