Months of mass anti-government protests led Abdel Mahdi to resign in December, but he has stayed on since in a caretaker role while political factions have struggled to find a successor.

Any contender would need the approval not only of the political class but also of Iraq's Shiite religious authority, neighbouring Iran, its rival the United States and the protest movement.

Two days ahead of the Saturday deadline, "President Barham Saleh is hosting the various political blocs to try to find a consensus candidate," a source from his office told AFP.

Saleh had sent a letter to Iraq's deeply-divided parliament, giving it until Saturday to nominate a new premier after months of paralysis. Otherwise, he said, he would name someone unilaterally.

As the clock was ticking, the United Nations' top official in Iraq ramped up pressure, too.

"It is high time to restore confidence by setting aside partisanship, acting in the interest of the country and its people," said Jeanine Hennis-Plasschaert in a statement.

"Iraq cannot afford the ongoing violent oppression nor the political and economic paralysis."

- Uncharted waters -

According to Iraq's constitution, parliament's largest bloc must nominate a prime minister within 15 days of legislative elections.

The candidate is then tasked by the president with forming a government within one month.

But Iraq is in uncharted waters, as the constitution makes no provisions for the PM's resignation and the 15-day period since Abdel Mahdi stepped down has long expired.

It is also unclear if the constitution allows the president to name his own candidate in this situation.

Saleh's letter sent factions into last-minute crisis talks, with one political figure saying they were "tense".

"They are divided over the old names that have been circulating in recent months and are even coming up with new names now," said the politician.

Possible successors that have been floated include an adviser to the president, an intelligence chief, political operatives and former top officials, among others.

One reason for the continued stalemate, a top government official told AFP, is the absence of Iranian general Qasem Soleimani and Iraqi commander Abu Mahdi al-Muhandis, who were both killed in a US drone strike in Baghdad on January 3.

The two influential figures had often taken the lead in brokering political agreements among parties.