At least 717 people have been killed and hundreds more injured outside Mecca in a stampede Thursday, as hundreds of thousands of Muslims participated in a ritual in the city of Mina.

Over two million Muslims are in Mecca on pilgrimage this week, celebrating the Feast of the Sacrifice, Eid al-Adha. The incident occurred during the “stoning of the devil” ritual, in which Muslims throw stones at a pillar, symbolizing the devil.

The disaster was the worst such stampede in 25 years, since back in July 1990 when 1,426 Muslim pilgrims were killed in a tunnel collapse near Mecca.

Officials are trying to determine how the stampede originated. According to Mansour al-Turki, head of the Saudi General Security Forces, it was a matter of “panic” as people crossed a bridge where the stones are hurled from.

"The trampling begins with a small number of people falling, followed by a state of panic and an attempt to escape the crowd, which leads to an increasing number of victims," news agencies quoted him as saying.

A Saudi minister responded by blaming "undisciplined pilgrims" for the stampede.

Fifteen Iranian nationals were killed in the stampede, prompting the Islamic Republic to accuse Saudi Arabia of safety errors. 

The head of Iran's hajj organization, Said Ohadi, claimed on Iranian state television that for "unknown reasons" a path had been closed off near the scene of the symbolic stoning of the devil ritual, which "caused this tragic incident."

"Today's incident shows mismanagement and lack of serious attention to the safety of pilgrims. There is no other explanation. The Saudi officials should be held accountable," he added.

There have been a spate of incidents in which pilgrims in Mecca were hurt or killed this year. On September 11, a crane toppled onto the Grand Mosque in Mecca, killing 118 and injuring 394. On September 23, over 200 Muslim pilgrims were trapped inside a train without air conditioning, causing dozens to faint and require hospital treatment.