Washington: A massive ground offensive involving 20,000 to 25,000 troops may be launched by the Iraqi forces as early as April to retake Iraq\'s second largest city of Mosul from the Sunni radical group Islamic State (IS), a US military officer has said.

The US Central Command (Centcom) officer said that the IS was on the defensive and that the plan against the militants is â€œslightly ahead of the campaignâ€, according to an ABC News report Thursday.

The official provided unusually detailed plans at a press briefing about the long-awaited push by the Iraqi forces to retake Mosul from the 1,000 to 2,000 IS fighters believed to there.

The plans for an offensive in April or May are ahead of predictions made last October by the US Centcom that an offensive might not occur for another year.

As many as 12 Iraqi Army brigades or between 20,000 to 25,000 troops would be involved in retaking the city that was seized by the IS in June last year, the official said.

An attack force of five Iraqi Army brigades or 10,000 troops would push into the city from the south with three other brigades making up a reserve force.

Furthermore, three brigades of the Kurdish Peshmerga forces â€œwill help contain (the IS) from the north and isolate (them) from the westâ€, said the official.

A â€œMosul fighting forceâ€ consisting mostly of former Mosul police officers and some tribal elements are currently being trained for stability operations after the main fighting occurs, US military officials said. Another brigade of elite counter-terrorism forces will also be involved in the offensive.

â€œThe five Iraqi brigades will all go through (preparation at) our training sites before we commence the operation on Mosul,â€ said the US Centcom official. The US has established five training sites for Iraqi and Kurdish brigades.

Telegraphing details of the offensive plan could be an element of psychological warfare against the IS, given the number of Iraqi troops that would take on a much smaller IS force in Mosul.

The US-led coalition airstrikes have taken such a toll on the IS that they could not regenerate the number of fighters being lost on the battlefield, the US military official said, noting that the militant group has sustained campaign losses in seven to eight months that were double of what the US has suffered in 14 years of combat in Iraq and Afghanistan.

â€œThere is no organisation in the world that can suffer those kinds of casualties and not have a tremendous impact on their ability to achieve their long term aims,â€ said the official, describing the IS as being in a â€œzero-sum gameâ€ where it could no longer position forces without having to take them out of other fighting positions.

â€œThey are losing ground in Iraq every day,â€ said the official, who noted that the IS was on the defensive in Iraq though small elements were still capable of conducting limited â€œmicro-offensivesâ€, as happened last week when the IS took over the town of al-Baghdadi in the western Iraqi province of Anbar.

Another key component of the strategy to combat the IS was the training of a force of 5,000 moderate Syrian rebels to fight the militants in Syria and Pentagon press secretary John Kirby said Wednesday that 1,200 potential trainees were being screened by the US Centcom and that training could begin as early as mid-March.

Massive push against IS in Mosul likely in April