An alliance led by nationalist cleric Moqtada Sadr won the biggest share of seats in parliamentary elections on May 12, the first polls held since the defeat of the Islamic State group.

The election saw a record number of abstentions as Iraqis snubbed the corruption-tainted elite that has dominated the country since the 2003 toppling of Saddam Hussein.

"The post-election phase represents a crucial time for Iraq," UN envoy Jan Kubis told a Security Council meeting on Iraq, calling for the swift formation of a new government.

"It is essential that the new government works as one across the sectarian and ethnic divides in pursuing much-needed political, economic and social reforms," he said.

The new government will face the daunting task of rebuilding the country, just five months after the defeat of the IS group.

Sadr is working to form a government of technocrats and has called for less foreign meddling, raising tensions with the United States and Iran.

US Ambassador Nikki Haley said the government formation talks represented a "key moment in Iraqi history."

"Iraq's next government is going to make a series of important decisions that will set Iraq's course for decades to come," Haley told the council.

The new government "will have to decide whether Iraq is serious about elevating female leaders" and set policies that "will allow Iraq to close the door on the extremism and the sectarian politics that have cause so much suffering before," she added.

The council last month postponed a planned visit to Iraq that had been proposed by the United States, at the request of the former government. No new date for a visit has been proposed, diplomats said.