At least 717 pilgrims from around the world were killed on Thursday in a crush outside the Muslim holy city of Makkah, Saudi authorities said, in the worst disaster to strike the annual Haj pilgrimage for 25 years.

At least 863 others were injured at Mina, a few kilometres east of Makkah, when two large groups of pilgrims arrived together at a crossroads on their way to performing the "stoning of the devil" ritual at Jamarat, Saudi civil defence said.

Thursday's disaster was the worst to occur at the pilgrimage since July 1990, when 1,426 pilgrims suffocated in a tunnel near Makkah. Both incidents occurred on Eid Al-Adha (Feast of the Sacrifice), Islam's most important feast and the day of the stoning ritual.

Photographs published on the Twitter feed of Saudi civil defence on Thursday showed pilgrims lying on stretchers while emergency workers in high-visibility jackets lifted them into an ambulance.

Other images showed bodies of men in white Haj garments piled on top of each other. Some corpses bore visible injuries.

Unverified video posted on Twitter showed pilgrims and rescue workers trying to revive some victims.

The Haj, the world's largest annual gathering of people, has been the scene of numerous deadly stampedes, fires and riots in the past, but their frequency has been greatly reduced in recent years as the government spent billions of dollars upgrading and expanding haj infrastructure and crowd control technology.

Safety during haj is a politically sensitive issue for the kingdom's ruling Al Saud dynasty, which presents itself internationally as the guardian of Islam and custodian of its holiest places in Makkah and Madinah.

Saudi Arabia's Crown Prince and Interior Minister Prince Mohammed bin Nayef bin Abdelaziz ordered a committee to be formed to investigate the disaster and present its findings to King Salman, the Interior Ministry said.

The ministry spokesman, Mansour Turki said the investigation would look into what caused an unusual density of pilgrims to congregate at the location of the disaster. "The reason for that is not known yet," he told a news conference in Mina.

Iranian state news agency Irna said at least 89 Iranians were among the dead and quoted Iran's Deputy Foreign Minister Hossein Amir Abdollahian as saying Saudi Arabia was responsible.

The semi-official Fars news agency reported that Tehran summoned the Saudi charge d'affaires to lodge an official complaint over the disaster.

South African Acting President Cyril Ramaphosa extended condolences to families of the victims and said his government was awaiting information about his country's pilgrims.

JAMARAT

Street 204, where the crush occurred, is one of two main arteries leading through the camp at Mina to Jamarat, the site where pilgrims ritually stone the devil by hurling pebbles at three large pillars. In 2006, at least 346 pilgrims died in a stampede at Jamarat.

"Work is under way to separate large groups of people and direct pilgrims to alternative routes," the Saudi Civil Defence said on its Twitter account.

It said more than 220 ambulances and 4,000 rescue workers had been sent in to help the injured. Some of the wounded were evacuated by helicopters.

An Arab pilgrim who did not want to give his name said he had hoped to perform the stoning ritual later on Thursday afternoon but was now too frightened to risk doing so.

"I am very tired already and after this I can't go. I will wait for the night and if it not resolved, I will see if maybe somebody else can do it on my behalf," he said.

Efforts to improve safety at Jamarat have included enlarging the three pillars and constructing a three-decker bridge around them to increase the area and number of entry and exit points for pilgrims to perform the ritual.

More than 100,000 police and thousands of video cameras are also deployed to allow groups to be dispersed before they reach dangerous levels of density.

"Please pilgrims do not push one another. Please leave from the exit and don't come back by the same route," an officer kept repeating through a loudspeaker at Jamarat.

Two weeks ago 110 people died in Makkah's Grand Mosque when a crane working on an expansion project collapsed during a storm and toppled off the roof into the main courtyard, crushing pilgrims underneath. - Reuters