Christian forces in Iraq have sworn to "fight relentlessly" to free Mosul and the surrounding areas from ISIS control.

In a statement released on Monday, the Nineveh Plain Forces (NPF) said: "we salute the Peshmerga forces, all Iraqi military forces, the federal police and the international coalition on the launch of the operations that will allow the retaking of the province of Nineveh".

Government troops backed by the US and other forces on Monday launched an air and ground offensive to recapture Mosul in the biggest operation since the Iraq invasion of 2003.

The city, once considered the heartland of Iraq's Christian population, was overrun by Islamic State in June 2014 and remains the jihadist group's last stronghold in Iraq.

The NPF said in its statement that it represents the "Chaldean-Syriac-Assyrian" people of Iraq, and is "honoured to participate in these operations to liberate all of our occupied territories, and to push out the terrorist gangs of ISIS.

"We have sworn to fight relentlessly, side by side with the allied forces, in order to free all of our cities and villages in the Nineveh Plain and Mosul area," the statement continued.

"We promise our people that we will honour this commitment."

The NPF is one of several Christian militias operating in Iraq. The Nineveh Plain Protection Unites (NPU) is another – the two belong to different political parties.

The NPU last month claimed to have driven out ISIS militants from Badaneh, a "traditionally Christian village" south of Mosul.

But though Assyrian Christians are among a number of religious minorities who have suffered immensely under Islamic State, there has been some disagreement within the Assyrian community as to whether so-called 'Christian militias' should exist at all.

Patriarch Louis Raphael I Sako said in May that supporting Christian forces would be "a bad idea".

"There are no 'Christian militias', but only politicized groups and simple people who are in desperate need of a salary," he said. "The remaining Christians in Iraq are only the poor and those belonging to the middle class, and among them, there are 100,000 displaced people."

The Assyrian Confederation of Europe (ACE), however, said it was concerned by Patriarch Louis Raphael I Sako's comments.

"There is broad agreement between Assyrian organisations in Iraq and the diaspora that Assyrians must actively participate in the military campaign to liberate the Nineveh Plain and secure the area after the liberation," ACE said.

"The Nineveh Plain Protection Units (NPU), which is tasked with that mandate, is officially recognised and supported by the Iraqi government."

On Tuesday, the armed forces closing in on Mosul said they had secured around 20 villages on the outskirts of the city.

Early reports suggest that forces are already stationed close to Qaraqosh and Bartalla, two Christian towns that have been under ISIS control for more than two years.

Qaraqosh was once home to Iraq's largest Christian community, with at least a quarter of the country's Christian population living in the city and its surrounding towns. Tens of thousands were forced to flee when militants moved in on August 6, 2014.

Up to 1.5 million are expected to flee Mosul in the coming days and weeks, which could make it the worst humanitarian crisis this year.