MINA, Saudi Arabia (AFP) - Sirens wailed and helicopters hovered overhead as dozens of ambulances brought in victim after victim from a stampede that killed more than 700 Thursday in the holy Saudi valley of Mina.

More than 800 people were injured in the worst tragedy to hit the annual haj in 25 years.

It happened as pilgrims carried out a symbolic ritual of stoning the devil.

At Mina Emergency Hospital, one of four facilities treating victims, a helicopter landed as ambulances arrived one after the other.

Injured pilgrims were brought in on stretchers, wearing chest badges giving their personal details, as security officers ushered away passers-by trying to gawk.

AFP reporters saw one woman carried in by four men.

Another apparently unconscious African woman was wheeled in the hospital as first aid teams rushed to check her pulse.

Other medical teams deployed to the stampede site at the Jamarat Bridge, a structure resembling a huge parking garage which hosts the stoning ritual and cost more than US$1 billion (S$1.4 billion) to build.

The five-storey structure is almost a kilometre long and allows 300,000 pilgrims an hour to carry out the stoning ritual.

The official Saudi Press Agency said more than 220 rescue vehicles had responded to the tragedy.

Amid the chaos as victims were treated, people with normal ailments were still arriving at the hospital.

One elderly pilgrim came in with two daughters who felt dizzy because of the high temperatures and scorching sun.

BODIES SCATTERED BY CRUSH

"Everybody is dizzy, hajji. Take them to any another health centre," a security officer shouted as two more ambulances arrived at once, bringing in more victims of the stampede.

While some pilgrims recounted news of the incident to each other, there were others who appeared completely unaware that there had been a tragedy.

Many men shaved their heads to end the state of purity known as ihram, in which they carry out the haj.

An Internet video showed bodies of pilgrims wearing their traditional white ihram attire left scattered by the crush, surrounded by discarded shoes, flattened water bottles and umbrellas that had been used for protection from the sun.

Nearly two million people from across the globe were attending the haj, one of the largest annual gatherings in the world.

Earlier on Thursday, AFP reporters heading back from a different part of the Jamarat Bridge saw a pilgrim who had almost fainted at stairs leading to an upper sector of Mina town.

Two companions sprayed her face with water and called for help.

Just a few steps away, Saudi policemen urged tired pilgrims resting on the sides of the stairs to get up and continue their journey, despite the heat.

Along the roads, security men sprayed passing pilgrims with water to cool them down.

"People were already dehydrated and fainting" before the stampede, said one Sudanese pilgrim who declined to be identified.

They "were tripping all over each other", he said, adding that a Saudi companion had warned him that "something was going to happen".