Washington (AFP) - The US wants Iraq to launch its offensive to retake the strategic northern city of Mosul from the Islamic State group in April or May, military officials said.

Mosul is believed to be held by 1,000-2,000 IS fighters and 20,000-25,000 Iraqi troops are needed to carry out the offensive, an official with US Central Command said on Thursday.

"The mark on the wall we are still shooting for is the April-May timeframe," the official said, adding that because of Ramadan and the increasing heat of summer, "it becomes problematic if it goes much later (than May)."

A US ground role in the offensive force, to help direct air strikes, has not been ruled out, the official said.

But Iraqi troops will form the bulk of the force, while three Kurdish brigades are planned to also participate.

US-led coalition aircraft have recently focused air strikes in the area of Mosul and Kurdish forces have made inroads on the ground nearby.

Kurdish peshmerga forces have also launched successful offensives against IS-held roads near Mosul, which is in the north of the country.

The city once held well over a million people but now is likely a fraction of that size.

Also Thursday, military chiefs from two dozen countries gathered in the Saudi capital to seek ways of bolstering the Iraqi army against the IS jihadists.

Many Iraqi soldiers abandoned their weapons and uniforms when IS advanced last June, seizing large areas of the country.

The extremists also hold parts of Syria.