MOSUL, Iraq, March 5 (Xinhua) -- Iraqi security forces on Sunday launched a new push toward Mosul old city center, on the western bank of Tigris River, amid heavy clashes with Islamic State (IS) militants, the Iraqi military said.

Early in the morning, the federal police and interior ministry special forces, known as Rapid Response, initiated a progress from the southern outskirts toward the IS defensive lines in the neighborhoods of Dandan and Dawassa, which are part of Mosul's old city center, Lt. Gen. Abdul-Amir Yarallah, from the Joint Operations Command said in a statement.

The battles in the old neighborhoods of downtown Mosul bring the troops closer to some main government buildings in Dawassa neighborhood.

Meanwhile, the commandos of the Counter-Terrorism Service advanced toward the neighborhoods of Sumoud and Tel al-Rumman in the southwestern part of the city, sparking fierce clashes with IS extremist militants, according to the statement.

The new push in the western side of Mosul, locally known as the right bank of Tigris, came after two-day pause of advance due to bad weather that had limited the air support by the Iraqi and international coalition aircraft.

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

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 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.