Iraq Prime Minister Haider al-Abadi has formally declared Mosul liberated from the Islamic State (IS) militant group after nine months of fierce fighting to dislodge the extremist militants from their last major stronghold in Iraq.

"I declare to the whole world the end, failure and collapse of Daesh state, the state of (IS group) terrorism, which they announced it here in Mosul three years ago," Abadi said in a speech in Mosul on Monday, Xinhua news agency reported.

On Sunday, Abadi, who is also the Commander-in-Chief of Iraqi forces, paid a visit to Mosul and held meetings with the military commanders before he made a tour in the eastern side of Mosul, where hundreds of people welcomed his presence.

Later on Sunday, Abadi said "the victory is guaranteed" and the remnants of IS group are "besieged in the last inches", according to a statement by his office.

On October 17, Iraqi Prime Minister Haider al-Abadi announced the start of a major offensive to retake Mosul, the country's second largest city.

Since then, the Iraqi security forces have inched to the eastern fringes of Mosul and made progress on other routes around the city, preparing for a major battle to storm the city and drive out IS militants.

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.

On February 19, the security forces resumed the offensive to drive extremist militants out of the western side of Mosul, locally known as the right bank of Tigris River which bisects the city.

However, the western part of Mosul, with its narrow streets and heavily populated neighborhoods of the Old City centre, appeared to be a bigger challenge to the Iraqi forces and took about five months until the troops fully liberated the while western side, including Old City.

Mosul, 400 km north of Iraq's capital Baghdad, came 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.