WASHINGTON — The fight for Mosul, launched this week by Iraqi security forces supported by U.S. air power and advisers, could take months and is likely to feature periods of fierce combat, the commander of U.S. forces in the Middle East said Wednesday.

Army Gen. Joseph Votel, commander of U.S. Central Command, said the duration of the battle will be determined in part by how the Islamic State group reacts and adjusts to the unfolding Iraqi offensive. He noted that the extremist group has had more than two years to prepare its defenses in and around the sprawling Tigris River city.

The militants thus far have put up significant resistance in villages surrounding Mosul. They have sent trucks loaded with explosives careening toward the front lines and fired mortars to slow the Iraqi forces’ advance since Monday. Iraqi soldiers are in the lead combat role; the U.S. is supporting them with a variety of aircraft, artillery and advisers but the Pentagon has said repeatedly that none of the Americans are on the front lines.

Speaking at the Center for American Progress, a Washington think tank, Votel said Iraqi officials have publicly spoken of the Mosul battle lasting for weeks or months. He said he is comfortable with the Iraqis’ timeframe and said it was important to credit the Islamic State with being an adaptive enemy capable of adjusting to setbacks.

“I’ll give you an example,” he said. “The operation we did to Manbij, in northern Syria, took about 71 days to complete, from start to finish.” That was completed in August. Votel noted that another key objective in Syria — Raqqa, the self-proclaimed IS capital — is about three times the size of Manbij.

“Mosul is about three times the size of Raqqa,” he said, suggesting that Mosul could be orders of magnitude harder than either Manbij or Raqqa. There had been speculation that offensives to retake Raqqa and Mosul might happen simultaneously. Ask about that, Votel repeatedly stressed the importance of what he called “simultaneous application of pressure.”

“I think it’s extraordinarily important and we are certainly attempting to do that,” Votel said. “I think what we’ve seen is when we apply pressure on the Islamic State forces, they do squirt out, they try to go to other locations, they move leaders, they move the bulk of their forces, they try to relocate some of their operations.”

He said the U.S. and its coalition partners could respond to that in various ways, including directing airstrikes at IS fighters seeking to move from Mosul to Raqqa.

“I think it’s extraordinarily important to apply pressure in many areas, Iraq and Syria,” he added.

Separately, another U.S. general said Islamic State fighters are likely to put up a stiff defense of Mosul but eventually lose their grip and morph into an insurgency.

Maj. Gen. Gary Volesky, commander of U.S. and coalition land forces in Iraq, said this transition from conventional combat to counter-insurgency is deemed so predictable that the U.S. training regimen for Iraqi security forces is already being adjusted to prepare them for insurgent threats.

Volesky, speaking to reporters at the Pentagon via video link from his headquarters in Baghdad, also disclosed that U.S. Army Apache helicopters have entered the battle for Mosul. He declined to provide specifics, citing the need to preserve operational security, but said they have been striking Islamic State targets at night. The mere presence of the Apaches on the battlefield has been a confidence booster for Iraqi soldiers, he said.

The Apaches, he said, can “see a long range at night” and strike targets from a great distance. “That’s what they’re doing,” he said.

Volesky said some Islamic State forces already are giving up their positions in the outskirts of Mosul and pulling back into the city. He said he expects this trend to continue. They are then likely to attempt to block the entry of Iraqi forces into the city, using a “full-fledged conventional defense.”

At some point, he predicted, the Iraqi forces will prevail, and at that point, “I expect they (Islamic State fighters) are going to go into insurgency mode.”

“That’s my assessment,” he added. “That’s what we’re preparing the Iraqis for.”

Earlier this month, a Canadian general who runs a portion of the coalition training of Iraqi security forces told reporters that retaking Mosul from the Islamic State would open a new, more dangerous phase of the counter-IS fight. Brig. Gen. Dave Anderson said the period between the fall of Mosul and the ultimate defeat of IS “is probably when it’s most dangerous.”

“Literally, what we’ve been talking about is how do we position police forces and minister of interior forces in order to be able to fight the enemy the day after Mosul and its new metastasized form,” Anderson said Oct. 5.

Associated Press writer Lolita C. Baldor and AP Radio correspondent Sagar Meghani contributed to this report.