A political showdown loomed in Baghdad as Nuri al-Maliki appeared determined Tuesday not to go down without a fight after his replacement as prime minister was internationally acclaimed.

Washington urged his successor, Haidar al-Abadi, to rapidly form a broad-based government able to unite Iraqis in the fight against rampant jihadist militants who have overrun large swathes of the country.

The United States, and other countries, said they were working to deliver much-needed arms to the Kurds, who are fighting the Islamic State (IS) on several fronts.

Abadi came from behind in a protracted and acrimonious race to become Iraq's new premier when President Fuad Masum Monday accepted his nomination and tasked him with forming a government.

He has 30 days to build a team which will face the daunting task of defusing sectarian tensions and, in the words of US President Barack Obama, convincing the Sunni Arab minority that IS "is not the only game in town".

"We are urging him to form a new cabinet as swiftly as possible and the US stands ready to support a new and inclusive Iraqi government and particularly its fight against ISIL (IS)," US Secretary of State John Kerry said in Sydney Tuesday.

He reiterated Washington's stance that US air strikes launched last week were not a prelude to the reintroduction of American combat forces.

After seizing the main northern city of Mosul in early June and sweeping through much of the Sunni heartland, jihadist militants bristling with US-made military equipment they seized from retreating Iraqi troops launched another onslaught this month.

They attacked Christian, Yazidi, Turkmen and Shabak minorities west, north and east of Mosul, sparking a mass exodus that took the number of people displaced in Iraq this year soaring past the million mark.

A week of devastating gains saw the jihadists take the country's largest dam and advance to within striking distance of the autonomous Kurdish region.

They also attacked the large town of Sinjar, forcing thousands of mainly Yazidi civilians to run up a mountain and hide there with little food and water.

US strikes and cross-border Kurdish cooperation appeared to stop the rot and yielded early results on several fronts, with thousands of Yazidis managing to escape their mountain death trap and Kurdish troops beginning to claw back lost ground.

The United States has been leading an increasingly international effort to deliver humanitarian assistance to the hundreds of thousands who have poured into Kurdistan over the past week alone.

US Defence Secretary Chuck Hagel said it was the Iraqi government that had requested US assistance in providing the peshmerga with more arms.

"We are -- American forces, through CENTCOM -- are helping get that equipment to Arbil," he said.

Obama had made it clear he thought no effective and coordinated anti-jihadist counter-offensive could take place while Maliki was still in charge.

But the 64-year-old Shiite leader appeared determined to pull every stop to stay in power for a third term.

Zaid al-Ali, an Iraqi lawyer and the author of The Struggle for Iraq's Future, said Maliki had some reasons to worry about his future if he relinquishes power.

"There's been so much blood, so much suffering over the past few years, he's going to be a marked man," he said.

Surrounded by 30-odd loyalists from his Shiite bloc, Maliki gave a speech denouncing Abadi's nomination as a violation of the constitution.

Maliki, who worked hand in glove with the US when it occupied Iraq, accused Washington of now working to undermine him.

But, even if he could still complicate the handover of power, he looked more isolated than ever, despite deploying special forces and armoured vehicles across strategic locations in Baghdad.

The UN's top envoy in Iraq called on the security forces to "refrain from actions that may be seen as interference in matters related to the democratic transfer of political authority".

"The only lasting solution is for Iraqis to come together and form an inclusive government," Obama told reporters on Monday.

While Maliki's Shiite militias and Iraq's armed forces have tried to battle IS fighters, the outgoing premier is seen by many as partly to blame for the crisis for having alienated the Sunni minority.

Further south, people in a Sunni Arab neighbourhood of the city of Baquba gathered in the street on Monday and fired shots in the air to celebrate Maliki's political defeat.

Abadi, a Shiite politician considered close to Maliki, was born in Baghdad in 1952 and returned from British exile in 2003 when US-led forces toppled Saddam Hussein.

"Up until recently, he's been a Maliki surrogate. I have never seen much daylight between the two of them," said Kirk Sowell, the Amman-based publisher of the Inside Iraqi Politics newsletter.