IRBIL, Iraq — U.S. commanders pressing for an attack on Mosul perhaps as early as this spring may be underestimating the importance of the city to its Islamic State occupiers, who are likely to put up a huge fight to retain control, experts who’ve studied the extremist organization say.

Iraqi officials have resisted what a U.S. Central Command briefer said last week were plans for the assault to begin in April or May and involve an Iraqi force of about 25,000.

In televised comments Tuesday night in Baghdad, Iraq’s defense minister, Khaled Obeidi, said it was irresponsible to alert the enemy to possible plans and that the decision on the timing of such an operation would be made by Iraqi officials in Iraq, not American officials in the United States.

But another aspect of the Centcom briefing has raised concerns among analysts: the briefer’s assertion that Mosul’s defense is in the hands of only 1,000 to 2,000 Islamic State fighters. That number underestimates how crucial Mosul is to the Islamic State’s key goal: building a caliphate that erases long-established borders and attracts the support of Muslims from throughout the world.