Iran turned its back on Nouri al-Maliki on Tuesday, and congratulated his designated successor as prime minister of Iraq, paving the way for Maliki's removal from power.

The move by Maliki's main sponsor, which was signalled by a senior official close to Iran's supreme leader, aligned Tehran with the US, Saudi Arabia and Nato in formally backing Haider al-Abadi as the new premier – apparently making Maliki's position untenable.

There were unconfirmed reports on Tuesday that Iran had offered the ousted prime minister asylum, while Maliki himself urged Iraq's security forces not to get involved in Baghdad politics.

The widespread international support for Abadi from across usual regional faultlines was expected to help him to form a broad-based government, which Maliki had persistently failed to do. It was unclear, however, what a show of political unity in Baghdad could do in the immediate future to contain the threat of the extremist Islamic State (Isis) group in the north.

Thousands of Yazidis remain trapped on Mount Sinjar in the north of Iraq, "without food, water or shelter", the UN said on Tuesday, as the west stepped up humanitarian support for those fleeing from the forces of the Islamic State.

US defence officials said that American planes were flying about 100 sorties a day over Iraq, with the aim of bringing help to the thousands of civilians of the Yazidi faith stuck on Mount Sinjar near the Syrian border and to curb the Isis threat to the Kurdish capital, Irbil, where there are a significant number of Americans. The US strikes targeted Isis artillery and mobile columns, providing a boost to Kurdish peshmerga forces, which recaptured two towns from Isis but lost a third.

According to the UN High Commissioner for Refugees (UNHCR), tens of thousands of Yazidis, a religious minority concentrated in northeastern Iraq, have escaped Sinjar and crossed into Syria and then back into the autonomous Iraqi region of Kurdistan, to the Dohuk governorate close to the Syrian border.

Adrian Edwards, a UNHCR spokesman said that after a mass influx in the past 72 hours, there are now 35,000 Yazidis and other minorities who escaped Sinjar taking refuge in Dohuk, where they were in dire need of food, water and shelter.

"As of now, an estimated 20,000 to 30,000 people remain trapped on Sinjar Mountain without food, water or shelter," Edwards added. "Access to these families is extremely limited."

Kurdish security chief Masrour Barazani said: "What happened in Sinjar was nothing less than an act of genocide. Women were enslaved, men were killed. We have sent additional troops to try to secure a route. But it is 170 kilometres away from the mainland of the Kurds. Anything we do we have to do through the Arab areas and they are often hostile."

Europe, including Britain, stepped up support for those fleeing, pledging more air drops, money and equipment and the European Commission pledged €5m (£4m) to help aid organisations across northern Iraq.

The Iranian announcement in support of Abadi was made by Ali Shamkani, the secretary of the National Security Council, who congratulated the Iraqi leadership and people on the appointment of the new prime minister. In what appeared a coordinated move the Iranian-backed Iraqi Shi'a militia, Asa'ib Ahl al-Haq (AAH), also swung its support behind Abadi, in a further sign that Maliki was running out of options.

Hossein Rassam, a London-based Iranian analyst, said Shamkhani's statement reflected Tehran's hand in Abadi's selection: "His appointment could not have materialised without Iran's cooperation. This is the result of a series of negotiations and bargaining for the past number of days, it's not something that has been decided overnight."

According to Rassam, Iran's top priority in Iraq has been to avoid a power vacuum in Baghdad and ensure the appointment of a prime minister sympathetic to Tehran. "With Abadi's appointment, Iran has achieved both," he said.

Anders Fogh Rasmussen, the Nato secretary-general said: "Forming such an inclusive government is essential in defeating the Islamic State in Iraq and the Levant [Isis], which represents a threat to the security and stability of the region as a whole."

The Iranian declaration of support overtly aligned Tehran with the US and Nato. Speaking in Australia, the US secretary of state, John Kerry, promised "additional political, economic and security options" once the new prime minister assembled a functioning government.

Kerry made clear there were no plans for a return of US combat troops. "There will be no reintroduction of American combat forces into Iraq," he said. "Nobody, I think, is looking forward to a return to the road that we've travelled."

However, on Tuesday night the defence secretary, Chuck Hagel, said the US was sending 130 more military advisers to Iraqi Kurdistan. Meanwhile a suicide bomber blew himself up on the checkpoint that leads to Abadi's home in Baghdad. There was immediate word on casualties.

The Vatican on Tuesday called on Islamic leaders to denounce unambiguously the persecution of Christians and Yazidis in Iraq. The department in charge of inter-religious dialogue said: "The dramatic situation of the Christians, the Yazidis, and other minority religious and ethnic communities in Iraq demands that religious leaders, and above all Muslim religious leaders, people engaged in inter-religious dialogue and all people of good will take a clear and courageous stance. All must be unanimous in their unambiguous condemnation of these crimes and denounce the invoking of religion to justify them."

An Iraqi military helicopter taking aid crashed on Mount Sinjar on Tuesday afternoon killing the pilot and wounding 20 others, including Iraq's only Yazidi MP, Viyan Dakhil, who was on board for a rescue mission.