Islamic State (Isis) have less than 10,000 fighters in the northern Iraqi city of Mosul, but it would be an "optimistic estimate" to say the city will be secured by the end of this year, a US official has said. But US Army Colonel Christopher Garver said the battle for the city had already started.

"We have struck [Isis] fighters, weapons, leaders, and financial assets with precision and lethality," the spokesman for the US-led Operation Inherent Resolve told a news conference on 4 March . But he said that Iraqi forces were responsible for planning for the operation with coalition support, although they were looking for ways to accelerate the timeline for its completion, which was slated for the end of this year.

This includes speeding up the training for the Iraqi soldiers, and providing options to help them on the ground, he added. Training also is underway for the police forces who will serve as the "hold force" throughout Iraq to prevent Isil's resurgence, he said.

With its narrow streets surrounded with homemade explosives or improvised explosive devices (IEDs), Garver said it would be difficult to uproot the Isis (Daesh) fighters in Mosul, he said, adding that morale was "necessarily good" for the group inside the city at the moment. However, he said militants were trying to send their families out of Mosul and US strikes on the banks oil facilities and banks had forced the group to pay less to its fighters.

When the moment arrives to take the city, he said that 8-12 Iraqi brigades along with two Kurdish Peshmerga brigades with around 2,000-3,000 troops each will lead the efforts to take the city.

His comments about the timeline echoed those given to the US House Armed Services Committee on 2 March by Marine Corps Lieutenant General Vincent Stewart, who said Iraqi forces were not ready to engage in the difficult, large-scale urban fight.

"Taking and securing Mosul in the next 8-10 months is not something I'm seeing in my crystal ball," Stewart said Wednesday. "They'll need a significant amount of help from coalition partners… I don't know they will ask for that help."

Elsewhere in Iraq, the country's forces are continuing to isolate the Iraq city of Fallujah where they have improved their defensive positions, Colonel Garver said. They have also retained their positions in Sinjar where thousands of Yazidi men were massacred by IS in 2014, he added.