WASHINGTON — Recent gains against the Islamic State in eastern Syria have helped sever critical supply lines to Iraq and set the stage for what will be the biggest fight yet against the Sunni militancy, the battle to retake Mosul, Pentagon officials said on Monday.
Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, said at a Pentagon news conference that American-backed forces had begun laying the groundwork for the fight by moving to isolate Mosul from the Islamic State’s de facto headquarters in Raqqa, Syria. Kurdish and Arab forces retook the town of Shaddadi in eastern Syria last week, cutting off what Defense Secretary Ashton B. Carter called the last major artery between Raqqa and Mosul.
But military officials cautioned that the fight for Mosul could last many months, requiring Iraqi forces unproven in urban warfare to advance street by street through the explosives-laden terrain of Iraq’s second-largest city, with more than one million people.
In addition to the advances in eastern Syria, the Pentagon has begun using cyberattacks on Islamic State communications between Raqqa and Mosul, as well as attacks meant to disrupt the militant group’s ability to use social media to recruit fighters, officials said.
Retaking Mosul would be a “massive hit” to the Islamic State, said Patrick Martin, an Iraq expert at the Institute for the Study of War. Such a loss would bolster claims by the American-led coalition that the Sunni militancy is on the run in Iraq. It could also sharply demoralize Islamic State fighters, raising questions about whether the group could still credibly call itself a caliphate.
The Pentagon has declined to predict when Iraqi troops will try to enter Mosul, though General Dunford said on Monday that “it is not something that will happen in the deep, deep future.” Prime Minister Haider al-Abadi of Iraq said two weeks ago that Iraqi forces would start a full military operation to retake the city as early as March, and an American military official said over the weekend that the Pentagon believed that Iraqi troops were ready to launch a credible assault.
Still, military officials acknowledge that the battle will be an uphill slog. “Do I think it’s going to be easy? No,” Maj. Gen. Richard Clarke, the commander of American land forces in Iraq, told reporters during a briefing last week. “It’s going to be tough.”
The long fight by Iraqi security forces to take back Ramadi from the Islamic State, which concluded in December, offers a preview of the battle to come over Mosul. Advancing inch by inch, Iraqi forces, backed by American airstrikes, took more than five months to gain control of the city center of Ramadi, the capital of Anbar Province.
As difficult as that battle was, the fight for Mosul will be much harder, military officials say. The city is five times as large as Ramadi. And while the Iraqi military used two American-trained brigades in the Ramadi fight — the 73rd and the 76th, which General Clarke said were believed to be the best in the Iraqi Army — those forces number some 8,000, far short of the 30,000 troops Pentagon officials say are needed.
Military officials also say it is hard to imagine how the fight for Mosul can be waged without close American air support, which would probably require American attack helicopters, something Mr. Abadi, for political reasons, has yet to agree to.
The effort is likely to include Kurdish pesh merga fighters, Pentagon officials say. The American military has trained some 16,000 Kurdish fighters, but their participation is likely to come with its own problems. “I do think the expectation is that the force will be heavily Kurdish, but then you get into the political issues,” said Kathleen H. Hicks, a former Pentagon official who is now at the Center for Strategic and International Studies.
Mr. Abadi’s government is unlikely to want the Kurdish fighters to assume the lead role in the coming fight, a role that Iraq experts say is likely to be filled by the Shiite-dominated Iraqi security forces.
In 2004, it took more than 13,000 highly trained troops, primarily Americans, almost two months to retake and clear Falluja of about 3,000 insurgents in the fiercest fight of the Iraq war. Ninety-five American service members died, and more than 560 were wounded.
The battle for Mosul, many military experts say, could be much worse. Pentagon officials say they are unsure how many Islamic State militants are in the city, but they have been there for almost two years.
“When the coalition cleared Falluja, it took forever, and Mosul is larger than Falluja,” Mr. Martin, the Iraq expert, said. “And the people who will be doing the clearing are not U.S. troops.”
Staff Gen. Talib al-Kinani, the commander of Iraq’s Counter Terrorism Service, said the units that would participate in the Mosul operation had already been chosen. Commanders have moved to a town on the edge of Kurdish-controlled areas to complete the final planning, he said.
General Kinani said the Iraqi military was working with Sunni tribes in Mosul against the Islamic State. The tribes, he said, “are giving information about those locations that are planted with explosive materials and I.E.D.s, and also they talk about the morale and the status of the ISIS fighters inside the city.”