"The whole world owes you a debt for your struggle against the deadly global threat posed by Daesh (IS)," Guterres told a donors' conference in Kuwait City in comments directed at the Iraqi delegation.

"It is time to demonstrate our lasting gratitude and solidarity with the Iraqi people," he said.

Guterres highlighted two key UN-backed initiatives, for which he was seeking support.

"The UN Development Programme's Funding Facility for Stabilisation is working in 25 cities and districts, supporting the return of displaced people to their homes," the UN chief said, adding that 2.5 million remained displaced.

He also announced a new, two-year Recovery and Resilience Programme.

"It aims to make immediate and tangible improvements to people's daily lives, rather than the long timelines associated with major infrastructure projects and economic reforms," he said.

Baghdad says it needs nearly $90 billion to rebuild after more than three years of fighting with IS devastated homes, schools, hospitals and other infrastructure.

Prime Minister Haider al-Abadi's government is hoping the international community will follow up the end of major combat against IS with concrete financial support for reconstruction.

The donor conference's host Kuwait pledged of $2 billion in investment and loans on Wednesday.