WASHINGTON — About More than one half of the entire 48,000-strong Iraqi Army will be dedicated to retaking the city of Mosul from IS fighters in sometime in April or May the April-May time frame, a senior CENTCOM official said on Thursday.

Approximately About 20,000 to 25,000 Iraqi and Peshmerga troops will move on the city to retake it from an estimated 2,000 IS fighters — an attacking force that will include five Iraqi Army brigades, three peshmerga brigades, and former Mosul police forces, tribal fighters, and Iraqi Counter Terrorism Service troops.

The official added that US planners have not "closed the door" on the possibility of placing US forward air controllers on the ground in Iraq to assist in calling in airstrikes on the densely populated city as Iraqi and peshmerga forces assault the deeply entrenched IS forces.

When it comes to the US-led air war over Iraq and Syria, US military planners estimate that IS has lost three quarters of a division worth of combat power over the last several months, and as many as 12,000 fighters have been killed.

While no hard numbers have been offered, the official estimated that IS deaths are "almost double in seven months what they have lost compared to 14 years in Iraq and Afghanistan for the US in total."

The US lost over 6,800 service members in Iraq and Afghanistan, meaning that IS has now lost somewhere in the vicinity of 12,000 fighters.

The Pentagon has been hesitant to place a body count on its operations in Iraq and Afghanistan, but just last month the US ambassador to Iraq estimated that about 6,000 IS fighters have been killed in coalition airstrikes. At the time, CENTCOM officials admitted that the number was likely accurate, but stressed that any number they offer is a vague accounting of after action reports, and not a detailed tally of actual numbers.

Still, the doubling of the tally of deaths is significant in that it reflects the devastating nature of the daily airstrikes that have been hitting IS positions in Iraq and Syria since August.

There are currently about 3,200 Iraqi army forces going through US-run training centers in Iraq, building on the 2,000 or so already trained, the official said. They are being trained by about 2,600 US troops at five training bases throughout the country, along with some Canadian, UK, Spanish, Danish and German special forces assisting in the effort.

The United States and allies have also given Iraqi forces about six brigades worth of equipment during over the last several months.

Significantly, the official also confirmed American awareness of Iranian troops in Iraq assisting local militias militas in fighting IS, saying that "we don't deny" that there is "Iranian activity and a force presence inside of Iraq, but thus far because we have a common goal and there's a common interest we have been working with the government of Iraq to make sure they understand that there are things out there that we cannot tolerate," such as like Iranian-influenced Shia militias abusing the Iraqi Sunni population.

"We don't have exact numbers but we know that there [is an] Iranian presence in Iraq," which the official said were "largely advisers" working with civilian militias.

Overall, the air war has been "generally unfolding as we had planned," the official said, stressing that degrading the ability of IS to capture ground remains a "multiyear effort."

Twitter: @paulmcleary