Iraq military Mosul
Volunteers from Mosul take part in military training as they prepare to fight against IsisReuters

A military force of 25,000 Iraqis and Kurds is being prepared to battle the Islamic State (Isis) in the northern Iraqi city of Mosul, a US official said.

The huge operation to retake Iraq's second biggest city will most likely take place in April or May this year, according to a senior figure in the US military who spoke to journalists on Friday (20 January).

Mosul, which was home to more than a million Iraqis before it was seized last June, is being controlled by between 1,000 and 2,000 IS militants, the official said.

Iraqi Prime Minister Haider al-Abadi said this week that Iraqi security forces were preparing an offensive to regain Mosul that would get underway in a matter of months, following an increasingly effective aerial campaign against the militants.

"I think the air campaign has increased in its quality and intensity," Abadi said.

IS has been on the defensive in recent weeks as the US-led coalition stepped up its attacks on the militants.

Nineteen villages in the Syrian province of Raqqa were recaptured from IS forces on Thursday with aerial support, according to UK-based watchdog Syrian Observatory for Human Rights.

With extensive assistance from a number of disaffected Sunni tribes in northern Iraq, IS routed Mosul in a matter of days in June 2014, when national security forces simply fled their posts.

IS has governed the city with little interference from the Iraqi government in the period, but militias and national security forces have challenged IS's rule in other parts of Iraq.

It had not yet been decided whether a small group of American advisers would be used to coordinate with aerial support, the US military official told reporters.

If Iraqi-Kurdish forces were able to retake the city, it would strike a major blow against the militants. It is the biggest city in their self-declared caliphate, which stretches across north-west Iraq and north-east Syria.