ARBIL, Iraq (AFP) - Iraq's elite counter-terrorism service (CTS) on Thursday (Oct 20) retook full control of Bartalla, a small Christian town to the east of Mosul which the Islamic State in Iraq and Syria (ISIS) group seized two years ago.

CTS commander Taleb Sheghati al-Kenani announced the militants' defeat in Bartalla on television from the town, which lies less than 15km from the eastern edge of Mosul.

"I announce to the people of Bartalla and Mosul we have complete control over Bartalla. Its residents, its churches and all of its infrastructure are now under the control of CTS," he said.

He stressed that control of Bartalla, which is now one of the spots where Iraqi forces are closest to Mosul, was key to the major offensive launched four days earlier.

Prime Minister Haider al-Abadi, who visited the front line on Wednesday, announced the launch of a broad offensive on Mosul early on Monday.

The operation is Iraq's biggest in years and aims to wrest back Mosul, the country's second city and the last major ISIS stronghold in Iraq.

"We tell the people of Mosul that we are coming to retake Mosul," Kenani said.

Bartalla is a predominantly Christian town that had a population of just under 10,000 people before ISIS swept across the Nineveh plain in August 2014, forcing everyone to flee.

Iraqi forces moving along several fronts have already retaken many villages - mostly east and south of Mosul - and hundreds of square kilometres of territory.

A million people or more are thought to be trapped in ISIS-held Mosul and surrounding areas.

Most are unable to leave, prompting fears the militants seek to use civilians as human shields, as they have done during previous assaults on their bastions.