MOSUL, Iraq, May 4 (Xinhua) -- Iraqi forces battling Islamic State (IS) militants on Thursday initiated a new front at the northwestern edge of IS stronghold in the western side of Mosul, the Iraqi military said.

The elite forces of the federal police, known as Rapid Response, and the army's 9th Armored Division advanced at 6:00 a.m. local time (0300 GMT) toward the areas of Mushairfah, Kanisah and Harmat at the northwestern edge of the western side of Mosul and fought heavy clashes with IS militants during the past hours of the day, the Joint Operations Command (JOC) said in a statement.

The new push would help the special forces of the Counter-Terrorism Service (CTS) and the interior ministry federal police, who are making slow progress in the southern part of Mosul's western side due to the stiff resistance of the extremist militants at the densely-populated areas of the old city center, where roughly 400,000 residents are believed to still be trapped under IS rule.

IS militants are now surrounded by the troops in the northern part of Mosul's western side, which includes the old city center.

Iraqi Prime Minister Haider al-Abadi, who is also the commander-in-chief of the armed forces, announced the start of an offensive on February 19 to drive extremist militants out of the western side of Mosul, locally known as the right bank of the Tigris River which bisects the city.

Late in January, Abadi declared the liberation of Mosul's eastern side, or the left bank of Tigris, after over 100 days of fighting IS militants.

However, the western part of Mosul, with its narrow streets and heavily populated neighborhoods, appears to be a bigger challenge to the Iraqi forces.

Mosul, 400 km north of Iraqi capital Baghdad, has been under IS control since June 2014, when government forces abandoned their posts and fled, enabling IS militants to take control of parts of Iraq's northern and western regions.