Top diplomatic and military officials from the US-led coalition fighting the Islamic State group met Thursday to prepare the assault on the city of Mosul, the jihadists' Iraqi bastion.

Defense and foreign ministers from more than 40 countries gathered in Washington for a second day as their local militia allies made advances in Syria.

While news from that front was dominated by the Syrian Democratic Forces' siege of Manbij, a city in northern Syria, the leaders in Washington were focused on a far bigger prize.

"Mosul will be the ultimate test," Brett McGurk, the US special presidential envoy to the Global Coalition to Counter ISIL -- another term for the IS group -- told the assembled delegates.

Backed by coalition air strikes and military advisers, Iraqi and Kurdish forces have made inroads in recent months into territory once controlled by IS group fighters.

But the daunting target of Mosul, Iraq's second city that is home to two million people, will be the campaign's center of gravity in the weeks and months to come.

"I am confident we are going to succeed, we are going to deprive Daesh of its geographical base," US Secretary of State John Kerry said after the talks concluded, using his preferred term for the group, an acronym formed from its Arabic initials.

"But Daesh will remain dangerous even when that defeat takes place," he warned.

On the first day of the talks, the defense ministers met separately to talk battlefield tactics while the foreign ministers pledged $2 billion in reconstruction funds.

The officials hailed the sum, but warned it may not be enough once the final battle is underway.

"We note with concern that military operations to liberate Mosul... and the possible displacement of up to one million people as a result, could increase humanitarian needs even beyond the recently pledged resources," their final statement read.

Thursday's meeting brought them together to discuss how to ensure that any victory in Mosul is quickly followed by a political settlement and the return of refugees.

"Let us remember, Mosul is where Abu Bakr al-Baghdadi declared his phony caliphate in June of 2014," McGurk said, referring to the IS group's elusive leader.

"And if we get this campaign right on the ground, in all its aspects, it is where we can begin to seal his fate."

The battle for Mosul and the Islamic State group's Syrian base Raqa will be tough if the jihadists decide to hold out in their symbolic strongholds.

But the Iraqi city also poses a political challenge.

- End of the 'caliphate' -

The remaining civilian population is mainly Sunni Muslim -- albeit with a historical Christian minority -- and distrustful of the Shiite-led government in Baghdad.

The city is also near the fracture line between Iraq's autonomous Kurdish region and the Arab center, a point of tension between the local anti-IS forces.

The challenge facing the ministers in Washington -- including Iraqis and Kurds -- was to agree on how post-war Mosul will be rebuilt and governed.

"Mosul will be the most complex operation to date," McGurk said, warning that a million civilians from many ethnic and religious groups remain inside the city.

Nevertheless, he added, the plan is "well underway" with an agreement on the type and number of Iraqi and Kurdish troops and militia to be used in the assault.

Critically, there has been an agreement that 15,000 locally recruited troops from Mosul's Nineveh province -- largely Sunni Arabs -- will be involved.

In light of the coalition's experience after the liberation of Tikrit, Ramadi and Fallujah, resources will be set aside for refugee camps and reconstruction.

"I believe thanks to the pledging conference yesterday and the meetings we're having this week that that foundation is being set," McGurk said.

"The liberation of Mosul -- and of Raqa -- is now an achievable objective and it's one we must get right."

US Defense Secretary Ash Carter and Kerry, who are co-chairing the meeting, said that driving the IS group out of Iraq and Syria was possible and necessary.

But they warned that although taking Mosul would deny the group space to train, plan and spread propaganda, it would not stop the jihadists from inspiring attacks far from their heartland.