Coalition-backed Iraqi forces can retake the key northern city of Mosul from the Islamic State group by the end of the year, the commander of US forces in the Middle East said Tuesday.

"It's the prime minister's objective to have that done by the end of the year," General Joe Votel, head of the US military's Central Command, told reporters in a video call.

"My assessment is that we can meet the prime minister's objective, if that is what he chooses to do."

The US-led coalition fighting IS in Iraq and Syria for the past two years has for months framed the upcoming battle for Mosul as the defining fight against the jihadists in Iraq.

Mosul is the country's second-largest city and is IS's center of operations in Iraq.

Iraqi security forces, with coalition support, have already started "shaping operations" around Mosul, including the capture of surrounding villages and facilities.

Votel said the fight for Mosul would be "hard and difficult" and that IS would leave countless bombs in the city while also hiding behind civilians.

Similarly, in Syria, coalition-backed local forces are beginning to hone in on Raqa, the self-proclaimed capital of IS's so-called "caliphate."

"We are at the point here where we are now really into the heart of the caliphate," Votel said.