BAGHDAD -- An Iraqi government-sanctioned paramilitary force moved on Monday to capture a key town beyond the city of Mosul from the Islamic State group, tightening its grip on series of towns and villages near the Syrian border, officials said.

Backed by the U.S.-led international coalition, Iraq last October launched a wide-scale military offensive to recapture Mosul and the surrounding areas, with various Iraqi military, police and paramilitary forces taking part in the operation. The city's eastern half was declared liberated in January, and the push for the city's western section, separated from the east by the Tigris River, began the following month.

According to Shiite lawmaker Karim al-Nouri, the mainly Shiite Popular Mobilization Forces seeks to drive IS militants out of the centre of strategic Baaj, west of Mosul near the border with Syria. Al-Nouri said the surrounding villages have already been taken from IS.

Once Baaj falls, he told The Associated Press, the fight with IS will move to the Syrian border. He didn't elaborate.

"Baaj is a strategical town for Daesh as it is the last supply line" linking IS with Syria, said Sheikh Sami al-Masoudi, a PMF leader, using the Arabic acronym for IS.

"Once we reach the border, we will erect a dirt barricade and dig a trench to derail their (IS) move," he added.

Hashim al-Mousawi, a leader with al-Nujaba militia, which is also part of the PMF, said the troops are ready to move inside Syrian territories but that this needs Iraqi government approval.

The Iran-backed PMF -- known as Hashed al-Shaabi in Arabic -- has largely operated since October in the desert to the west of Mosul, trying to cut IS supply lines.

In Mosul, Iraqi forces began a new offensive to drive IS militants from the remaining pockets of territory that the militants still hold in the Old City, in Mosul's western half. The IS hold on Mosul has shrunk to just a handful of neighbourhoods in and around the Old City district where narrow streets and a dense civilian population are expected to complicate the fight.

Iraq's second-largest city, Mosul fell to IS in the summer of 2014 as the militants swept over much of the country's north and central areas. Weeks later the head of the Sunni extremist group, Abu Bakr al-Baghdadi, announced the formation of a self-styled caliphate in Iraq and Syria from the pulpit of a Mosul mosque.