WASHINGTON (Reuters) - Iraq and the United States have launched a crucial battle to liberate the city of Mosul without determining how its volatile region will be governed once Islamic State militants are ejected, U.S. and other officials said.

U.S. officials acknowledge gaps and risks in the plan for Mosul, amid worries that defeat of Islamic State in its de facto Iraqi capital could give way to sectarian score-settling and land grabs in the country’s ethnically mixed north.

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But they argue that the alternative -- waiting to first sort out Iraq’s fractious sectarian politics -- is unrealistic. With Islamic State hurting militarily, now is the time to strike, they say.

Plans for administering Mosul itself, and aiding hundreds of thousands of civilians who could flee the fighting are in place, Western and Iraqi officials say.

But being left for later, they say, are fundamental issues likely to determine Iraq’s future stability. Among them are bitterly contested territorial claims in the country’s north, including the divided city of Kirkuk and the disputed borders of the Kurds’ autonomous region.

In Mosul, it remains to be seen how power will be shared among the city’s Sunni Arabs, Kurds, and minority Turkmen, Christians, Yazidis and others.

“Some of those big-picture governance, territorial issues, are going to be pushed down the road,” a senior State Department official said.

Lukman Faily, Iraq’s ambassador to Washington from 2013 until earlier this year, said that while military planning is advanced, “on the politics, we still need to get our house in better shape.”

The United States has repeatedly found in recent years that the aftermath of war can prove more troublesome than the fighting itself.

It invaded Iraq in 2003 without a detailed post-war plan and with insufficient troops, contributing to the chaos that still engulfs the country more than 13 years later. In Afghanistan, the Taliban are making gains 15 years after U.S. and allied Afghan forces ousted them from Kabul.

Peshmerga forces advance in the east of Mosul to attack Islamic State militants in Mosul. REUTERS/Thaier Al-Sudani

Iraqi government forces, backed by air and ground support from the U.S.-led coalition, on Monday launched the initial stages of the offensive to retake Mosul. The assault has been in preparation since July.

Fighting is expected to take weeks, if not months, as government forces, Sunni tribal fighters and Kurdish Peshmerga first encircle the city of more than 1 million and then attempt to oust between 4,000 and 8,000 Islamic State militants.

“If we try to solve everything before Mosul, Daesh will never get out of Mosul. And this is really a war of momentum,” Brett McGurk, U.S. President Barack Obama’s counter-Islamic State envoy, told reporters this month. Daesh is a derogatory Arabic term for Islamic State.

At stake for Obama is his hoped-for legacy of seizing back as much territory as he can from the jihadists before he leaves office in January. The launch of the Mosul campaign comes three weeks before the U.S. presidential election on Nov. 8.

“There is a desire to make as much progress against Daesh as possible,” said a Western diplomat speaking on condition of anonymity.

UNEASE

The decision to back Iraqi Prime Minister Haider al-Abadi’s move on Mosul, which involves a force of more than 30,000 fighters, has support in key quarters of Obama’s administration.

But some U.S. defense and intelligence officials question whether Iraq’s rebuilt army is ready. And they say they worry that the aftermath of a messy battle could be a political nightmare as Sunnis, Kurds and Shiites each try to hold parts of the city they have helped liberate.

“In some circles around Washington, they want ... things completely in place before the military campaign starts,” said an administration official, speaking before Abadi’s announcement of the start of the offensive early on Monday.

“Taking Iraqis’ focus away from the military fight to resolve all the longstanding political fights will only achieve a loss of momentum against ISIL,” the official said, using another acronym for the group.

Other spoilers are possible, officials acknowledge.

Turkey, which has longstanding cultural and historic ties to Mosul, says a force it has trained in northern Iraq is now participating in the fight. Powerful Shi’ite militias also want a role, raising fears of sectarian clashes in majority Sunni Mosul.

In weeks of intense regional diplomacy, senior U.S. envoys drove home the message that all forces must be under Abadi’s command, the officials said. Whether the message sticks remains to be seen.

While bigger issues will be left for later, plans have been laid for governing and stabilizing Mosul in the near-term after the fighting subsides, the officials said.

The plan calls for the governor of Mosul’s Nineveh province, Nawfal al-Agoub, to be restored and the city divided into sub-districts with local mayors for each. Agoub will govern along with a senior representative from Baghdad and from Erbil, capital of Iraq’s autonomous Kurdish region.

Screening procedures for civilians fleeing Mosul have been enhanced, in an effort to learn from the battle for Fallujah, in Anbar province. There, Sunni men and boys were held, tortured and in some cases killed by Shi’ite militia members, who had erected makeshift checkpoints.

U.S. and Iraqi officials are working to ensure displaced civilians take safe routes out of the city, and that checkpoints are overseen by provincial authorities and monitored by international non-government groups.

They are also hoping that Mosul’s populace stay in their homes if possible, unlike in the cities of Tikrit, Fallujah and Ramadi, which virtually emptied out as they were being freed from Islamic State’s grip.

The United Nations has said the Mosul battle could leave as many as 1 million people homeless.

“There’s a chance, maybe a significant chance, that it’s going to be fewer people than we expect, but of course it would be dangerous to assume that,” the senior State Department official said.

The Western diplomat acknowledged that moving now against Islamic State’s stronghold in Mosul involves a balancing act.

“There’s a certain amount where you can prepare as much as possible, but once you sort of hit the ground and things actually start happening, you actually have to be quite flexible and ready,” the diplomat said.