SANAA - Yemen's ex-president Ali Abdullah Saleh was killed Monday as fighting raged between his forces and Iran-backed rebels, the insurgents and his party said, following the dramatic collapse of their anti-government alliance.

Saleh , who ruled Yemen with an iron fist for three decades until 2012, had joined forces with the Shia Huthi rebels three years ago when they took control of large parts of the Arabian Peninsula country including the capital Sanaa.

But that alliance unravelled over the last week, with heavy fighting across the capital, and Saleh was reportedly shot dead by Huthi fighters after he fled the city.

His death could mark a major turning point in a conflict that has left thousands dead, led to one of the world's worst humanitarian crises and deepened tensions between Middle East rivals Saudi Arabia and Iran.

President Abedrabbo Mansour Hadi, the head of Yemen's internationally recognised government, moved to take advantage of the chaos by ordering an offensive to retake the capital.

The Saudi-led military coalition backing Hadi's government carried out a wave of air strikes on the capital and warned residents to evacuate areas under rebel control. The Huthi-controlled interior ministry in Sanaa announced Saleh 's death on the rebels' Al-Masirah television station.

A statement read on the channel announced the "end of the crisis of militias", referring to Saleh 's armed supporters, and "the killing of their leader and a number of his criminal supporters".

A video provided to AFP by the rebels showed what appeared to be a dead Saleh with a severe head injury, his body wrapped in a floral-print blanket. Armed men could be seen loading the body into the back of a pick-up truck in an empty, sandy lot. Saleh 's political party, the General People's Congress, confirmed the death of the 75-year-old, blaming the rebels. "He was martyred in the defence of the republic," said Faiqa al-Sayyid, a party leader.

Sayyid said Saleh and other top party officials had came under Huthi gunfire as they fled Sanaa. A military source said the Huthis stopped their four-car convoy about 40 kilometres (25 miles) south of Sanaa and shot dead Saleh and two other senior GPC officials.

The collapse of the alliance between Saleh and the rebels saw at least 100 people reported dead in fighting, accusations of betrayal and the former leader reaching out to the Saudi-led coalition. The fighting continued on Monday, with reports of heavy clashes and coalition strikes against Huthi-controlled government buildings and around Sanaa International Airport.

The government, which has operated out of southern city Aden since being ousted from the capital, ordered an offensive to advance on Sanaa. "The president has ordered Vice President Ali Mohsen al-Ahmar, who is in Marib (east of Sanaa), to activate military units and advance towards the capital," a presidency official told AFP, speaking on condition of anonymity.

Military and government sources said the army would advance on Sanaa from the east and northeast, with at least seven battalions ordered to move forward. The government also reached out to Saleh 's supporters with an offer of amnesty. "The president will soon announce a general amnesty for all those who collaborated with the Huthis in recent months and who have retracted that allegiance," said Prime Minister Ahmad Obaid bin Daghr.

The Saleh-Huthi alliance had been fraught since its inception in 2014, when the two ended decades of enmity and joined ranks to capture Sanaa from Hadi's government. Saudi Arabia, accusing Iran of backing the rebels, intervened in the Yemen war on behalf of the government the following year.

Saleh on Saturday announced he was open to talks with Saudi Arabia and its allies on condition they ended their crippling blockade of Yemen's ports and airports. That dealt a serious blow to his already fragile alliance with rebel chief Abdul Malik al-Huthi. In a televised speech on Monday, Huthi made no mention of Saleh 's death but expressed his satisfaction at the day's events. "Today marked the failure of the conspiracy and treason, a black day for the forces of aggression," he said on Al-Masirah.

The fresh violence has increased fears for civilian victims of Yemen's war, which has claimed more than 8,750 lives since the Saudi-led coalition intervened. The conflict has pushed Yemen to the brink of mass starvation and triggered what the United Nations has called the world's worst humanitarian crisis.

International aid groups warned on Monday they were losing the ability to reach civilians in Sanaa. "Ambulances and medical teams can't access injured, people can't buy food and other supplies," UNICEF's Rajat Madhok said on Twitter. "Aid workers can't travel and implement critical life-saving programmes. This latest violence couldn't come at a worse time."