Since American air power will be critical to helping the Iraqi and Kurdish forces advance, the main question Mr. Obama will have to answer is whether the challenges posed in retaking Mosul mean that teams of American joint terminal attack controllers, or JTACs, need to be on the ground so that the airstrikes can be delivered precisely.

These teams, if deployed, would most likely need to be protected by Special Operations forces. There would also need to be additional quick-reaction forces held in reserve for emergencies, as well as medical personnel and helicopters in case the Americans came under heavy fire, former commanders said.

Although Mr. Obama has sent Marines and Special Forces to train Iraqi and Kurdish troops and advise them at the brigade level, he has not approved their use on the battlefield to call in airstrikes or advise Iraqi forces in combat. Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, has said he would ask the president for such authorization if needed.

In preparation for the assault on Mosul, the United States and its allies are trying to weaken the Islamic State by cutting its supply lines. Kurdish forces, backed by American-led air power, have recently positioned themselves near an important crossroads at Kiske, 25 miles west of Mosul. “The isolation of Mosul is going on now,” said the official at Central Command.

In addition, American officials took the unusual step on Thursday of announcing the timing of the battle and the number of Iraqi and Kurdish forces to be deployed. Openly discussing future military operations is normally off-limits to avoid aiding the enemy, but American officials said it was done this time to try to weaken the resolve of the Islamic State fighters and to spur Mosul’s residents to rise up against the occupiers and help the Iraqi ground forces.

That strategy angered Mr. McCain, the chairman of the Senate Armed Services Committee, who, with Senator Lindsey Graham, Republican of South Carolina, sent a blistering letter to Mr. Obama on Friday denouncing the briefing for reporters. “Never in our memory can we recall an instance in which our military has knowingly briefed our own war plans to our enemies,” Mr. McCain and Mr. Graham wrote.