ISIS has executed 25 people in Mosul, northern Iraq, by lowering them in a vat of nitric acid, according to several local news reports.

The men had been accused of spying on ISIS on behalf of Iraqi government security forces.

According to witnesses, the 25 alleged 'spies' had been tied together with a rope and lowered in a large basin containing nitric acid until their organs dissolved.

Brutal: The 25 men had been tied together with a rope and lowered in 'a large basin containing nitric acid' in Mosul, Iraq, after ISIS accused them of spying for the government (file photo)

'ISIS terrorist members executed 25 persons in Mosul on charges of spying and collaborating with Iraqi security forces,' a source told Iraqi News in a statement.

'ISIS members tied each person with a rope and lowered him in the tub, which contains nitric acid, till the victims organs dissolve.'

Nitric acid is a colourless, yellow or red, fuming liquid with an acrid, suffocating odour which is highly corrosive to all parts of the human body.

It is normally used in manufacturing ammonium nitrate for fertilizer and explosives, organic synthesis, photoengraving, etching steel, and reprocessing spent nuclear fuel.

The executions in Mosul follows a number of deadly bombings in the capital Baghdad, as ISIS hopes to rebound from a series of battlefield losses in Iraq.

Losing ground: Fighters from the Iraqi pro-government forces flash the sign for victory as they drive vehicles in the Albu Aitha area, north of Ramadi, Iraq, where ISIS has lost more than half of its territory

The terrorist group has continued losing control over territory across Iraq and Syria, a U.S. Military spokesman said this week, including almost half of what it had once held in Iraq.

The U.S. Defense Department had previously estimated that ISIS fighters had lost control of about 40 per cent of the territory they claimed in Iraq and about ten per cent of the land they held in Syria.

Those tallies had gone up in recent weeks, Pentagon spokesman Peter Cook said.

'The number right now in Iraq is about 45 per cent of the territory they once held has been recovered,' Cook said.

'The number in Syria is anywhere between 16 to 20 per cent.'

ISIS stormed across large parts of Iraq and Syria in early 2014, meeting little resistance from Iraqi security forces and exploiting the chaos in civil-war-torn Syria.

Since August 2014, the United States has led an international coalition fighting back against the IS group, using a combination of air strikes and training and equipping local partners.

ISIS has now lost control of Ramadi and Heet in Iraq, but still control other important cities including Mosul and Fallujah.