BAGHDAD — An Iraqi and Kurdish military force of some 20,000 to 25,000 troops is being prepared to recapture the city of Mosul from “Islamic State” fighters, probably in the April-May time frame, an official at the U.S. Central Command said on Thursday.

The official said Mosul was currently being held by 1,000 to 2,000 “Islamic State” fighters. No decision has been made on whether small numbers of U.S. military advisers might need to be on the ground close to Mosul to direct close air support, the official told a group of reporters on condition of anonymity.

Mosul, which had a population of more than 1 million people, was captured by “Islamic State” fighters in June and is the largest city in the group’s self-declared caliphate, a stretch of territory that straddles the border between northern Iraq and eastern Syria.

The main attack force being assembled for the Mosul campaign would include five Iraqi army brigades, the official said. Three smaller brigades would act as a reserve force and three brigades of Kurdish peshmerga troops would contain the city from the north and isolate it from Islamic State forces further west.

A so-called Mosul fighting force consisting mainly of former Mosul police officers and tribal fighters also is being assembled for the assault, the official said. About a brigade of counterterrorism troops also would be employed in the fight, he said. The total number of troops would be 20,000 to 25,000.

The aim of Iraqi and U.S. military leaders is to have all five Iraqi army brigades that comprise the main attack force participate in U.S.-led training currently going on in Iraq. That will begin once the present batch of 3,200 trainees rotate out of the five training sites, the official said.