The parliament delayed the session, the official INA press agency reported, without saying whether a new date had been agreed.

The agency reported earlier that speaker Haibat al-Halbussi received requests from Iraq's two main Kurdish parties, the Kurdistan Democratic Party (KDP) and the Patriotic Union of Kurdistan (PUK), to postpone the vote to "allow both parties more time" to reach a deal.

By convention, a Shiite Muslim holds the powerful post of prime minister, the parliament speaker is a Sunni and the largely ceremonial presidency goes to a Kurd.

Under a tacit agreement between the two main Kurdish parties, a PUK member holds the Iraqi presidency, while the president and regional premier of the semi-autonomous Kurdish region is selected from the KDP.

But this time the KDP named its own candidate for Iraq's presidency: Iraqi Foreign Minister Fuad Hussein.

Once elected, the president will then have 15 days to appoint a prime minister, expected to be former premier Nouri al-Maliki.

On Saturday, the Coordination Framework, an alliance of Shiite parties with varying ties to Iran that holds a parliamentary majority, endorsed Maliki.

But his nomination appeared to stoke concern in Washington.

The 75-year-old shrewd politician is Iraq's only two-term premier (2006-2014) since the 2003 US invasion.

Seen as close to Iran, Maliki left power in 2014 following heated pressure from Washington.

US Secretary of State Marco Rubio warned on Sunday against a pro-Iranian government in Iraq.

An Iraqi source close to the Coordination Framework told AFP that Washington had conveyed that it "holds a negative view of previous governments led by former prime minister Maliki."

In a letter, US representatives said that while the selection of the prime minister is an Iraqi decision, "the United States will make its own sovereign decisions regarding the next government in line with American interests."

Another Iraqi source confirmed the letter, adding that the Shiite alliance had still moved forward with its choice, confident that Maliki could allay Washington's concerns.

Iraq has long been a proxy battleground between the US and Iran, with successive governments negotiating a delicate balance between the two foes.

Iraq's new premier will be expected to address Washington's longstanding demand that Baghdad disarm Tehran-backed factions, many of which are designated terrorist groups by the US.