BAGHDAD, Oct. 31 (Xinhua) -- The Iraqi security forces on Monday started an operation to enter the eastern side of the city of Mosul as part of a campaign to drive out the Islamic State (IS) group from their last major stronghold in Iraq, a military statement said.

Iraqi army and counter-terrorism forces started the operation at 6:30 a.m. (0330 GMT) when the troops advanced in three routes toward the eastern side of the Tigris River, which bisects the city, the statement said without giving further details as fierce battles are underway.

During the past two weeks, the Iraqi security forces had inched to the eastern fringes of Mosul, and were making progress on other routes around the city preparing for a major battle to storm the city and drive out the IS militants.

On Oct. 17, Iraqi Prime Minister Haider al-Abadi, who is also the commander-in-chief of the Iraqi forces, announced the start of a major offensive to retake Mosul, the country's second largest city.

Mosul, some 400 km north of the Iraqi capital of Baghdad, has been under IS control since June 2014, when Iraqi government forces abandoned their weapons and fled.