Russia foils terror attack by ISIS on Moscow as FSB arrests 4 people. [Representational photo]Reuters

A military operation to liberate the Iraqi city of Mosul from the Islamic State group has begun, Prime Minister Haider al-Abadi declared in an address on state television. The city has been under the control of the terrorist group since June 2014.

The Kurdish Peshmerga and the Iraqi forces have launched an offensive against Isis with the backing of the United States-led coalition forces as well as British and French forces. Mosul, the second largest city of Iraq, has been surrounded by a 30,000—strong force for several days.

"We started fighting Isis in the outskirts of Baghdad, and thank God we are now fighting them in the outskirts of Mosul, and God willing the decisive battle will be soon... These forces that are liberating you today, they have one goal in Mosul which is to get rid of Daesh and to secure your dignity. They are there for your sake," PM al-Abadi said in his address to the country.

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

State TV broadcast patriotic music following the announcement by the prime minister, the Guardian reported. Sporadic artillery was also heard.

Thousands of leaflets were dropped across the city ahead of al-Abadi's announcement warning citizens to avoide certain parts of Mosul declaring that it was time for victory.

Iraqi forces joined the Kurdish Peshmerga soldiers and took their final positions on Friday. The US, British and French forces have also joined hands and will call in airstrikes against Isis inside Mosul. The fight is expected to last weeks if not months.

Isis reportedly has around 6,000 militants ready to defend their stronghold. Most of the civilians, expected to be around 600,000, might flee the city once the battle intensifies. The city had a population of around 2 million people when the terrorist group captured it.

The International Committee of the Red Cross and the United Nations raised the issue of safety of the civilians and have warned that the battle could result in a humanitarian crisis with almost a million refugees fleeing the city.

"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," Stephen O'Brien, UN deputy secretary general for humanitarian affairs and emergency relief, said. He also warned that "families are at extreme risk of being caught in cross-fire or targeted by snipers."

Civilians have been banned from fleeing the city by the group that has set up checkpoints on roads and blown up the houses of those who fled the city, the daily reported.

If the civilians stay in the city, they become victims of airstrikes, street battles and a seige by the forces. Isis might also use them as human shields to protect themselves. If they decide to leave, they would have to trek through minefields and there is also the risk of being found out and punished by the Isis.