Mosul, March 20 (IANS): Iraqi government forces battling the Islamic State retook control of more areas in the old city centre of western Mosul, while an international airstrike on the IS positions killed six leaders of the extremist group in the city, the Iraqi military said on Sunday.

The commandos of the Counter-Terrorism Service (CTS) fought heavy clashes against the IS militants during the day and freed the neighbourhood of Nablus in west of the old city centre and raised the Iraqi flags over some of its buildings, Abdul-Amir Yarallah from the Joint Operations Command (JOC) said in a statement, Xinhua reported.

The elite CTS special forces are also fighting the IS militants in several nearby neighbourhoods, as they have been advancing closer to the western edge of Mosul's densely populated old city centre, where hundreds of thousands of civilians are believed to still be trapped under the IS rule.

Meanwhile, the progress by the federal police and Rapid Response brigades, the elite Interior Ministry units, was slowed by bad weather and fierce resistance of the IS militants in the newly-freed Bab al-Toub neighbourhood and adjacent areas.

Also in the day, the US-led international coalition conducted an airstrike and killed six IS leaders in Souq al-Shaareen area in the IS-held city centre in western Mosul, locally known as the right bank of the Tigris River, which bisects Mosul, the JOC statement said.

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.

Late in January, Abadi declared the liberation of the eastern side of Mosul, or the left bank of Tigris, after more than 100 days of fighting against IS militants.

However, the western part of Mosul, with its narrow streets and a population of between 750,000 and 800,000, appears to be a bigger challenge to the Iraqi forces.

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