Iraq's premier designate Haidar al-Abadi is gaining widespread support from countries hoping political reconciliation will undercut jihadists, as Iran on Tuesday further undercut Nuri al-Maliki's bid to cling to power.

While the political drama unfolded, many thousands of members of minority groups in north Iraq, including Yazidis and Christians, faced a major threat from militants of the Islamic State (IS) jihadist group and a worsening humanitarian situation.

The UN said that 20,000 to 30,000 people were still stranded on a mountain after fleeing the fighters.

A helicopter carrying aid to trapped people crashed during takeoff in the north, killing a pilot and injuring Yazidi MP Vian Dakhil, who has worked to bring attention to the plight of besieged members of her community.

Washington is urging Abadi to rapidly form a broad-based government able to unite Iraqis in the fight against the jihadists, who have overrun swathes of the country.

The United States and other countries have also said they are working to deliver much-needed arms to the Kurds, who are fighting IS militants on several fronts.

Britain said it has agreed to transport military supplies for the Kurdish forces from "other contributing states."

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" IS, US Secretary of State John Kerry said in Sydney Tuesday.

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

In a further blow for Maliki, Iran on Tuesday ended its long-time support for him and swung its allegiance behind Abadi in a congratulatory message.

"We congratulate Haidar al-Abadi on his nomination as prime minister, for him personally and for religious dignitaries, the Iraqi population and its political groups," Ali Shamkhani, secretary and representative of supreme leader Ayatollah Ali Khamenei on Iran's Supreme National Security Council, said in Tehran.

- Time of crisis -

The political transition comes at a time of crisis for Iraq.

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 captured 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 sent the number of people displaced in Iraq this year soaring.

A week of devastating gains saw the IS 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 hide on Mount Sinjar with little food and water.

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

Dakhil, who had made an emotional appeal in parliament on behalf of members of her Yazidi community besieged on Mount Sinjar, was injured when a helicopter delivering aid crashed during takeoff, senior army officers said.

A pilot was killed in the crash, while New York Times journalist Alissa J. Rubin was injured, the paper said.

UN refugee agency spokesman Adrian Edwards told reporters there were 20,000-30,000 people on Sinjar Mountain and UN minority rights expert Rita Izsak warned they face "a mass atrocity and potential genocide within days or hours."

The United States has been leading an increasingly international effort to deliver humanitarian assistance to the hundreds of thousands who have poured into the Kurdish region in north Iraq.

- Maliki bid at dead end -

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

While Maliki insists the premiership should be his, declaring Abadi's selection a "constitutional violation," his bid to retain power has reached a dead end with the widespread international backing for his rival, especially from Tehran and Washington.

Maliki on Tuesday ordered the armed forces to "stay away from the political crisis", assuaging fears that he could seek to leverage military power to stay in office.

In an apparent warning to Maliki, US State Department spokeswoman Marie Harf said Tuesday that Washington "would reject any effort, legally or otherwise, to achieve outcomes through coercion or manipulation of the constitutional or judicial process."

"There's a constitutional process, it is happening, and that is what we support."