The United States urged members of the coalition fighting Islamic State on Tuesday to help rebuild Iraq or risk the reversal of the gains made against the group there.

The U.S. leads the coalition and hopes that after a three-year fight to defeat the militants it can count in large part on Gulf allies to shoulder the burden of rebuilding Iraq and on a Saudi-Iraqi rapprochement to weaken Iran's influence in the country, which is run by a Shi'ite led government.

Islamic State took control of large parts of Iraq and Syria in 2014.

Donors and investors have gathered in Kuwait this week to discuss efforts to rebuild Iraq's economy and infrastructure as it emerges from a devastating conflict with the hardline militants who seized almost a third of the country.

Iraq declared victory over Islamic State in December, having taken back all the territory captured by the militants in 2014 and 2015. The fighters have also been largely defeated in neighboring Syria.

The U.S. appreciates the "generous contributions" of coalition members over the past year but more is needed, Secretary of State Rex Tillerson told a coalition meeting in Kuwait.

"If communities in Iraq and Syria cannot return to normal life, we risk the return of conditions that allowed ISIS to take and control vast territory," he said, using an acronym to refer to the group.

"We must continue to clear unexploded remnants of war left behind by ISIS, enable hospitals to reopen, restore water and electricity services, and get boys and girls back in school."

The U.S. is not expected to make a direct government contribution at the conference, however.

"It's not in the question of a pledging thing where we go out with requests, it's underscoring there is a need for support. It's investment, it's private company engagement, it's DFI," a senior state department official traveling with Tillerson told reporters on Monday.

The term DFI generally refers to development finance institutions such as multilateral development banks.

"But it is also and there are Arab donors willing to help and support focus your efforts on those areas where it actually achieves something in terms of stabilization and countering foreign influence."

Rebuilding Iraq after three years of war will cost more than $88 billion, with housing a particularly urgent priority, Iraqi officials told the conference on Monday.

Tillerson also said Washington had decided to provide an additional $200 million of aid to stabilize liberated areas in Syria.

At the same time, NATO Secretary-General Jens Stoltenberg said the alliance was ready to answer a U.S. call for it to expand its small training mission in Iraq to support reconstruction.

U.S. Secretary of Defense Jim Mattis sent a letter to NATO last month calling for a formal NATO train-and-advise mission, Reuters reported, part of President Donald Trump's campaign for the alliance to do more against militants.