Iraqi forces thrust into west Mosul from the north on Thursday, opening a new front in the more than six-month-old offensive to dislodge the Daesh terrorist group from the country's second city.

The assault is aimed at sealing the siege of the Old City, where die-hard jihadists holding huge numbers of civilians hostage are preparing for a bloody last stand.

Forces from the army, the interior ministry and the police "began breaching the western side from the north," the Joint Operations Command (JOC) coordinating the war against Daesh said in a statement.

"Now your sons are fighting and striking the enemy's defenses... They rejoice in victory or martyrdom for the sake of liberating the rest of the city of Mosul from Daesh terrorists," it said.

The JOC said the targets of the latest push were northwestern neighborhoods on the edges of Mosul called Musharifah, Kanisah and Al-Haramat.

The federal police issued a statement confirming the new operation, which comes after a relative lull in fighting around the Old City where most remaining jihadists are believed to be holed up.

It said Iraqi forces had already captured a small outlying village called Hsunah and a nearby gas factory.

Thursday's operation opens a new front in the effort to wrest back west Mosul from Daesh which started in mid-February and saw thousands of Iraqi forces retake most southern and western neighborhoods.

An advance from the north will eventually leave the jihadists completely trapped in the narrow streets of the Old City, albeit with a large civilian population they have been using as human shields.

The United Nations said up to 400,000 people might be trapped in the densely populated Old City, which lies just west of the Tigris River that divides Mosul. The Iraqi immigration ministry said this week that the number of people who were displaced since the start of the offensive on Mosul on October 17 had topped 600,000.

The vast majority of them fled homes on the west side of the Tigris, where the fighting has been more intense than in east Mosul, which was declared retaken by the government in January.

Of the more than 120,000 people displaced from east Mosul, the ministry said only 42,000 were still in camps.

The recapture of Mosul by the Iraqi security forces would deal a death blow to the "caliphate" Daesh proclaimed over large parts of Syria and Iraq nearly three years ago.

According to the authorities, Daesh now controls less than seven percent of Iraq, down from more than a third of the national territory in 2014.

The militants have in recent days carried out diversionary attacks against security forces from their desert hideouts in the western province of Anbar. At least 26 members of the army, police, tribal forces and border guard died in the attacks but the militants' "state" has looked doomed for months.