A military operation to recapture the Iraqi city of Mosul from so-called Islamic State (IS) has begun, Prime Minister Haider al-Abadi says.

The long-awaited assault from Kurdish Peshmerga, Iraqi government and allied forces is backed by the US-led coalition fighting IS in Iraq.

Mosul, Iraq's second largest city, has been under IS control since June 2014.

The UN has expressed "extreme concern" for the safety of up to 1.5 million people in the area.

Mosul is the group's last major stronghold in Iraq. The loss of the city, officials say, would mark the effective defeat of IS in the country.

Sporadic artillery was reportedly heard in areas around the city after the announcement.

In an address broadcast on state television in the early hours of Monday (local time), Mr Abadi said: "The hour of victory has come, the operation to liberate Mosul has started."

"Today I declare the start of the heroic operations to liberate you from Daesh," he said, using another name for IS.

"God willing we will meet in Mosul to celebrate the liberation and your salvation from Isis (IS) so we can live together once again, all religions united and together we shall defeat Daesh to rebuild this dear city of Mosul."

Surrounded by senior Iraqi officers, he vowed that only government forces would enter Mosul, a Sunni-majority city.

This, analysts said, was an attempt to counter fears that the operation could turn into a sectarian conflict.

US Secretary of Defence Ash Carter described it as "a decisive moment" in the campaign to deliver IS "a lasting defeat". But he added that it would be a "difficult fight".

It was from Mosul that IS leader Abu Bakr al-Baghdadi declared a caliphate - a state governed in accordance with Islamic law - in territory controlled by the group in Iraq and Syria.

An operation to retake the city, capital of the northern Nineveh governorate, has been planned for months, and analysts say it could last for weeks, if not months.

Thousands of pro-government forces have gathered at an airbase in the strategic town of Qayyarah, about 60km (37 miles) south of Mosul, which was recaptured in August.

Thousands of leaflets have been dropped warning residents that the offensive was imminent.

How will battle unfold? By Michael Knights, Washington Institute for Near East Policy

What can be said with certainty is that the liberation of Mosul will be a multi-phased operation.

First the logistical base for the operation must be established at Qayyarah airbase. This is also the collecting point for the Iraqi forces that will liberate Mosul.

The next phase will be a multi-pronged advance on the outskirts of Mosul. This phase will unfold in fits and spurts: one day 10 miles will be gained easily, another day there will be tough fighting at an IS strongpoint or a pause to bring up supplies.

During November and December the main battle will likely begin. But for the Iraqi and coalition forces the issue of civilians will be a tricky factor.

The UN under-secretary for humanitarian affairs and emergency relief coordinator, Stephen O'Brien, asked for civilians to be protected and be given access to assistance "they are entitled to and deserve".

"I am extremely concerned for the safety of up to 1.5 million people living in Mosul who may be impacted," he said in statement.

As many as one million people could be forced to flee their homes because of the operation, he added.

There are no firm figures on how many people remain in Mosul, but there were more than two million there when IS took it more than two years ago.