At least 717 pilgrims were killed on Thursday in a stampede outside the Muslim holy city of Mecca, Saudi authorities said, the worst disaster to strike the annual haj pilgrimage in 25 years.

At least 805 others were injured in the crush at Mina, a few kilometers east of Mecca, caused by two large groups of pilgrims arriving together at a crossroads on their way to performing the "stoning the devil" ritual at Jamarat, Saudi civil defence said.

The pilgrimage, the world's largest annual gathering of people, has been the scene of deadly disasters in the past, including stampedes, tent fires and riots.

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

The government has spent billions of dollars upgrading and expanding haj infrastructure and crowd control technology in recent years, and the last big deadly incident took place in 2006 when at least 346 pilgrims died in a stampede.

Street 204 is one of the two main arteries leading through the camp at Mina to Jamarat, where pilgrims ritually stone the devil by hurling pebbles at three large pillars. Jamarat was also the site of the 2006 disaster and several other past ones.

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.

Reuters reporters in another part of Mina said they could hear police and ambulance sirens, but that roads leading to the site of the disaster had been blocked.

Ambulances

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

It said more than 220 ambulances and 4,000 rescue workers had been sent to the stampede's location to help the injured. Saudi-owned Al-Arabiya television channel showed a convoy of ambulances driving through the Mina camp.

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

Unverified video posted on Twitter showed bodies, clad in the white towelling of those undertaking haj, lying on the ground by the side of the road, surrounded by debris, as pilgrims and rescue workers attempted to revive them.

Efforts to improve safety at Jamarat have included enlarging the three pillars and constructing a triple-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 Mecca'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.