WASHINGTON » U.S. intelligence agencies and the Pentagon are struggling to determine how difficult it will be to retake Mosul, the Islamic State’s de facto capital in Iraq, as planning intensifies for a battle that is becoming a major test of the Obama administration’s strategy to stop the spread of the terrorist group in the Middle East.

The assessment will be pivotal in driving important policy and military decisions that President Barack Obama will need to make in the coming weeks, including whether the Pentagon will need to deploy teams of U.S. ground forces to call in allied airstrikes and advise Iraqi troops on the battlefield on the challenges of urban warfare.

Reclaiming Mosul, which has a population of more than 1 million people and is Iraq’s second-largest city, will require 20,000 to 25,000 Iraqi and Kurdish forces to clear it block by block, with many of the streets and buildings likely to be rigged with explosives, U.S. officials said. The battle is planned for as early as April.

The city is being held by 1,000 to 2,000 Islamic State militants, according to U.S. military estimates. It sits astride one of the major infiltration routes that the Islamic State, also known as ISIS or ISIL, has used to ferry troops and supplies into northern Iraq from Syria.

U.S. intelligence agencies say they do not yet know whether Islamic State fighters will dig in and defend Mosul to the death or whether, fearing encirclement, most Islamic State fighters will slip out of the city for other Iraqi towns or cross the border into Syria, leaving behind a smaller force and booby-trapping buildings with bombs to tie down and bloody thousands of Iraqi troops.

"We are looking at all the things that are out there, i.e., what is the final enemy disposition in Mosul?" said a U.S. Central Command official who briefed reporters at the Pentagon on Thursday. The briefing drew sharp criticism from Sen. John McCain, R-Ariz.

The official, who spoke on the condition of anonymity to discuss future operations, continued, "All those things will have to be considered in the final analysis, and then ultimately they will go to the president with those things, and he will make that decision." Central Command oversees U.S. military operations in the Middle East, and officials there are helping Iraqis in the war planning.

The plan to retake Mosul, which the Islamic State has controlled since June, faces an array of challenges. The strategy is to draw on five of the most experienced Iraqi army brigades, about 10,000 troops in all, put them through several weeks of special training and then use them in conjunction with Kurdish peshmerga units and other forces to mount the main assault. But both U.S. and Iraqi commanders have raised doubts about the readiness of Iraq’s ground forces, which have struggled to recapture smaller towns that pose far less of a challenge than Mosul.

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

These teams, if deployed, would 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 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, 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 U.S.-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, U.S. officials took the unusual step 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 U.S. 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 Islamic State occupiers and help the Iraqi ground forces.

That strategy angered McCain, chairman of the Senate Armed Services Committee, who with Sen. Lindsey Graham, R-S.C., sent a blistering letter to 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," McCain and Graham wrote.

A defense official said Friday that the White House and Ashton B. Carter, the new defense secretary, "had no advance knowledge" of the briefing. Carter, speaking to reporters Friday en route to Afghanistan, gave no date for the assault on Mosul. "Even if I knew exactly when that was going to be, I wouldn’t tell you," he said.

U.S. officials said it was possible that announcing the battle for Mosul ahead of time could prompt many of the Islamic State fighters to slip away and make the retaking of the city less of a fight – perhaps to the point that it might not be necessary to have Americans call in airstrikes and advise Iraqi troops on the battlefield. But the officials acknowledged they did not know how the militants would respond. Some experts who recently visited Iraq, however, said the Islamic State’s actions in the towns of Kobani in Syria and in Baghdadi in Iraq’s Anbar province did not suggest that the fighters would flee. They said the Islamic State would not only try to hold parts of Mosul but would also launch diversionary attacks against the Iraqi forces elsewhere.

"They are going to fight back hard," said Kenneth M. Pollack, a former CIA analyst now at the Brookings Institution. "They are not only going to try to hold Mosul but will try to relieve the pressure by launching counteroffensives elsewhere."

Michael Knights, an expert on Iraq at the Washington Institute for Near East Policy, said the Islamic State’s strength was in the western half of the Mosul, which has important government buildings and is predominantly Arab. There are more Kurds in eastern Mosul. "My gut is that they continue doing an economy-of-force effort, screening and lively raiding, on the east bank until the Kurds get serious and push forward," Knights said. "Then they fall back to the west and blow the bridges."

"They’ll fight like devils for west Mosul, making the entire place into a huge harassment minefield full of unexploded car bombs, roadside IEDs and masses of fake IEDs," he added.

The battle, he said, could be a slugfest, reminiscent of the epic 2004 battle for Fallujah in the Iraq War, unless local residents or neighboring tribes took matters into their own hands, or the Americans and Iraqis had an effective covert program to undermine the Islamic State’s defenses from within.

Another challenge will come if the city is retaken. While Mosul is overwhelmingly Sunni, the Iraqi attacking force is likely to be overwhelming Shiite, which may create friction with the local population. The Sunni Arab population could be alienated if their neighborhoods were held by Shiite-dominated units or peshmerga forces.

The plan calls for introducing a stabilizing force that would be composed of former Mosul police officers and Sunni tribal fighters. But it is unclear how synchronized this effort will be.

"It may well be the Iraqi forces are ready to assault Mosul in a couple of months," said one European military officer who had been briefed on some of the battle planning. "But what comes next, who holds the ground taken, that could be just as difficult."

Michael R. Gordon and Eric Schmitt, New York Times

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