Operations to retake Mosul, Iraq's second biggest city, from the self-declared "Islamic State" (IS) group have begun, Prime Minister Haider al-Abadi announced early Monday on state television.

"The time of victory has come and operations to liberate Mosul have started," he said in an address broadcast by the Iraqiya channel. "Today I declare the start of these victorious operations to free you from the violence and terrorism of Daesh (IS)," he said, addressing residents of the Mosul region. 

"The force leading liberation operations is the brave Iraqi army with the national police and they are the ones that will enter Mosul, not others," al-Abadi added.

In a Facebook post, the prime minister wrote: "I announce today the beginning of the operation to liberate the province of Nineveh."

The operation began with a month-long buildup. For the last week, the final urban stronghold of IS in Iraq has been almost completely surrounded by a 60,000-strong force.

The widely-anticipated military offensive will be the biggest military operation since US troops left the country in 2011.

Iraqi Kurdish peshmerga forces have been moving in from the eastern side of the city, while a US-led coalition has provided support in the air and on the ground.

US Defense Secretary Ash Carter called the operation "a decisive moment in the campaign." He said the US and other members of the international coalition "stand ready to support the Iraqi security forces, peshmerga fighters and the people of Iraq in the difficult fight ahead."

Brett McGurk, the US envoy for the global coalition to counter IS, issued a short statement via Twitter early on Monday morning.

Airstrikes will 'not target civilians'

The Iraqi army dropped thousands of leaflets over Mosul before dawn on Sunday, warning residents that the operation to recapture the city was in its final stages of preparation. They said army units and airstrikes would "not target civilians."

IS has held the country's second-largest city since mid-2014 and may have 6,000 fighters on the ground ready to defend it.

As the operations began, the United Nations expressed concern for civilians in Mosul. "I am extremely concerned for the safety of up to 1.5 million people living in Mosul who may be impacted by military operations to retake the city from ISIL," said Stephen O'Brien, UN deputy secretary-general for humanitarian affairs and emergency relief, using another acronym for IS.

He warned that "families are at extreme risk of being caught in crossfire or targeted by snipers."

Russian President Vladimir Putin said Sunday he hoped the United States and its allies would do their best to avoid civilian casualties in the attack.

jm/cmk (Reuters, AFP)