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More than 700 people died and at least 800 were injured in a stampede at the Hajj pilgrimage outside the holy city of Mecca on Thursday, Saudi Arabian officials said.

It was the deadliest incident to hit the annual Hajj in years and the second this season, coming less than two weeks after 107 worshipers were killedwhen strong winds toppled a crane at Mecca's Grand Mosque.

Saudi Arabia's civil defense directorate said the crush happened at around 9 a.m. local time (2 a.m. ET) after there was a "sudden rise" in the number of pilgrims traveling along a major road leading to the site of a stone-throwing ritual.

This "resulted in a stampede among the pilgrims and the collapse of a large number of them," the directorate said in a statement. Emergency services were immediately dispatched and officials prevented any more pedestrians from heading to the area, it added.

The U.S. Embassy said on Twitter that it knew of no Americans killed or injured.

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The cause of the crush was not immediately clear. Saudi Health Minister Khaled al-Faleh said in a statement that authorities were investigating whether it began after some pilgrims failed to follow the signposted directions, although he did not provide any evidence to support this theory.

In the hours after the tragedy, the death toll steadily climbed. By 3:30 p.m. local time, the number of dead stood at 717 and the number of injured at 805.

More than 220 rescue vehicles and 4,000 people were helping respond to the incident.

Saud Al Sharif, a government employee, was nearby when the stampede broke out.

"It was insanely busy and people were coming from both directions," the 30-year-old told NBC News.

"People were climbing over one another just to breathe," said Abdullah Lotfy of Egypt. "It was like a wave. You go forward and suddenly you go back."

Images of the chaotic aftermath showed dozens of bodies lying in the streets, some of their faces covered in the white garments traditionally worn for the pilgrimage. Emergency vehicles lined the road and officials in high-visibility jackets attempted to tend to the injured.

The annual Hajj pilgrimage — which began on Tuesday — attracts around 2 million Muslims to Mecca.

Mina is the site where pilgrims carry out the "stoning of the devil," a ritual part of the hajj where people throw pebbles against three walls. The stampede occurred along the road leading to the stone-collection point.

Thursday's tragedy was the latest stampede to mar the Hajj and surrounding events in recent years. In 2006, more than 360 people were killed in a stampede in Mina, according to The Associated Press. Crushes and stampedes killed 244 people in 2004, 35 in 2001, 180 in 1998, and 270 in 1994, the AP reported.

The deadliest incident occurred in 1990, when a stampede in an overcrowded pedestrian tunnel leading to Mecca's holy sites saw 1,426 people die, according to the news agency.