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 Arab League's general secretariat condemned the Iran-aligned Houthi movement which killed Saleh as a "terrorist organisation" and demanded that the worldwide community view it as such.
United Nations spokesman Stephane Dujarric said Cheikh Ahmed called Saleh's killing "an adverse development" which would constitute "a considerable change to the political dynamics in Yemen".
Both Saleh forces and the Houthi rebels are battling to capture the capital city. "He got what he deserved", Ali Akbar Velayati, an aide to Iran's supreme leader, was quoted as saying by the semi-official Tasnim news agency.
Saleh's forces were key to helping the Houthis overrun Sanaa in 2014, and then much of the north and center of the country.
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.
Saleh, who ruled Yemen with an iron fist for three decades until 2012, had joined forces with the Shiite Huthi rebels three years ago when they took control of large parts of the Arabian Peninsula country including the capital Sanaa.
Yemen's pro-Houthi Al Masirah television station said the coalition bombed Saleh's residence and other houses of his family members.
A Saudi-led coalition has been fighting Iranian-backed Houthi rebels in Yemen for more than two years.
The killing of Yemen's ex-President Ali Abdullah Saleh by the country's Shiite rebels as their alliance crumbled has thrown the almost three-year civil war into unpredictable new chaos.
The proxy war between regional arch-rivals Saudi Arabia and Iran has already killed more than 10,000 people, with more than 2 million displaced.
Saleh's slaying likely gives the rebels the upper hand in the dayslong fighting for the country's capital, Sanaa.
At least 234 people have been killed and more than 400 wounded in fighting in Sanaa since the beginning of the month, the International Committee of the Red Cross (ICRC) said on Tuesday. Witnesses said the bodies of slain civilians and fighters littered the streets as ambulances were unable to reach them.
The war and blockade has plunged Yemen into a major humanitarian disaster, leaving 20 million people in need of aid. Speaking to reporters by phone from Sanaa, he said that "at the same time, people are bracing themselves for more".