Some analysts noted that most if not all of the Iraqi cities ravaged during the fight against the Islamic State, also known as ISIS, ISIL and Daesh, are in predominantly Sunni areas. Those were the places where a Sunni insurgency took root after the American-led invasion of Iraq in 2003, and later gave rise to an affiliate of Al Qaeda and the leadership ranks of the Islamic State. If they are not rebuilt, diplomats fear the region’s populace will once again turn to extremism, a risk Mr. Tillerson highlighted in a morning speech to a separate gathering of the 74-nation coalition to defeat the Islamic State.

“The end of major combat operations does not mean that we have achieved the enduring defeat of ISIS,” he said. “ISIS remains a serious threat to the stability of the region, our homelands and other parts of the globe. Without continued attention on the part of coalition members, we risk the return of extremist groups like ISIS in liberated areas in Iraq and Syria and their spread to new locations.”

Approximately 2.5 million Iraqi citizens remain displaced after their homes were flattened, utilities destroyed and schools bombed. Almost half of Mosul remains uninhabited.

The Trump administration has vastly reduced American aid for refugees and resettlement, arguing that helping displaced people return to war-torn regions is preferable to resettling them in the United States or elsewhere. But no one can return without intense rebuilding efforts.

Mr. Trump complained Monday that the United States had spent $7 trillion in the Middle East over the past 17 years with little to show for it.

“Seven trillion dollars. What a mistake. But it is what it is,” he said in introducing his domestic infrastructure plan.

Mr. Trump’s calculation included the direct costs of wars in Iraq and Afghanistan, as well as estimates for the costs of care for veterans. Whatever the total, the Pentagon has spent the vast majority of the money, not the State Department.