By Warren Strobel, Jonathan Landay and Stephen Kalin

WASHINGTON/BAGHDAD (Reuters) - Dozens of defense and foreign ministers will meet in Washington on Wednesday and Thursday to take stock of the fight against Islamic State, their focus increasingly on a major prize: the militant group's bastion in Mosul, Iraq.

The battle for Mosul is expected to be difficult, but the aftermath could be tougher, Iraqi, United Nations and U.S. officials say. Plans are still being finalized to provide urgent humanitarian aid and restore basic services and security for residents and as many as 2.4 million displaced people.

Defense ministers of the anti-Islamic State coalition will meet at Joint Base Andrews outside Washington on Wednesday, followed by a joint session of foreign and defense chiefs on Thursday.

The United Nations is preparing for what it says will be the largest humanitarian relief operation so far this year as terrified people stream out of the path of the advancing Iraqi military and flee from the city itself. They will need shelter, food and water, and sanitation for three to 12 months, depending on the extent of the city's destruction.

"There is a logic in moving as quickly as possible, but there is a danger that if the humanitarian response is not as prepared ... then we could have a humanitarian catastrophe and possible problems with political management of Mosul after its liberation," said a senior diplomat based in Baghdad, speaking on condition of anonymity.

The vast majority of the expected refugees will be Sunni Muslims, many of whom feel disenfranchised by Iraq's Shi'ite-led government in Baghdad, and that presents what could be an even bigger problem.

"Unless underpinning an offensive on Mosul are real political settlements between the Sunnis and the Shia, we think it’s only a matter of time before it unravels again," said a source in the Kurdish regional security council.

Iraqi Prime Minister Haider al-Abadi plans to install a military governor for Mosul after Islamic State is expelled, several sources said.

A U.S. defense official, speaking on condition of anonymity, questioned whether Iraq's military can retake the city without "prolonged and substantial help" from Kurdish security forces and Shiite militias. That sectarian mix could further complicate attempts at post-conflict reconciliation.

"ISIL will lose regardless of who goes in," the Kurdish security source said, using a common acronym for Islamic State. "What's important isn’t a military defeat; what’s important is the Iraqi government’s ability to embrace post-ISIL management issues, one of which is the Sunni grievances in and around Mosul. They’ve got to address that before the offensive."


Mosul, which Islamic State seized from a collapsing Iraqi army in June 2014, is Iraq's second biggest city and home to a combustible mixture of Sunni Arabs, Kurds, Turkmen and others.

Although Iraqi and U.S. officials have not announced a timetable for moving on the city, a senior Baghdad-based diplomat said Abadi wants to advance the start of the Mosul campaign to October after the seizure of the city of Falluja from Islamic State last month.

This month, Iraqi forces backed by U.S. air power retook Qayara air base south of Mosul, which will be turned into a logistics hub for the main assault on the city.

"We're looking ahead to Mosul, which will be the most significant challenge yet," Brett McGurk, U.S. President Barack Obama's special envoy in the fight against Islamic State, said on Tuesday.

McGurk said he met recently with Iraqi officials in Erbil to discuss the "disposition of forces" for the battle. Troops will include Kurdish peshmerga fighters, the Iraqi military and 15,000 local fighters from Nineveh province, he said.

McGurk described three other challenges this week's meetings will address in detail: plans for immediate humanitarian relief; short-term stabilization of Mosul; and local governance.

When asked whether he thought Islamic State would put up a strong fight in Mosul, Iraqi foreign minister Ibrahim al-Jaafar said he expected them to behave as they did in Falluja

"In Falluja they threatened and vowed to fight until the last breath ... issues ended differently; some were killed, some were defeated early and some disguised as women to flee," Jaafari told journalists in Washington.

The U.S. defense official said there are differences within the American military over the timetable.

"It makes sense, as (commanding U.S. Lieutenant General Sean) MacFarland is arguing, to capitalize on ISIS's recent setbacks by moving on Mosul and Raqqa this fall," said the official. Raqqa is the group's Syrian capital.

"The trouble is, ‘Then what?’, and it’s complicated by the fact that if you envelop the city, that leaves ISIS no way to retreat as they did from Falluja, and it could make for an even longer, nastier and more destructive fight if a lot of them decide to martyr themselves there," he said.

Lise Grande, the deputy U.N. representative in Iraq, said in a telephone interview: "We understand that there could be accelerated plans for Mosul, and we don't know what those plans are, but we have to be ready for them."

The United Nations says it needs an immediate $280 million to begin pre-positioning supplies - tens of thousands of tents and hundreds of mobile health clinics, for example - for the expected flood of refugees.

An Iraq donor meeting of 24 countries in Washington on Wednesday is expected to raise more than $2 billion, a senior State Department official told reporters on Monday.

"We hope it's more than this, but $2 billion now and maybe in the future other things can be added to this, that's what we seek," Jaafari said.

The U.N. estimates that under a worst-case scenario, more than 1 million people could be displaced from Mosul and another 830,000 from a populated corridor south of the city, adding to the burden of caring for the 3.5 million Iraqis displaced by Islamic State’s 2014 onslaught and U.S.-backed Iraqi counter-offensives.

(Additional reporting by Yara Bayoumy and John Walcott in Washington; Editing by James Dalgleish and Andrew Hay)