Blame shifted towards Saudi authorities Friday after a crush at the hajj killed at least 717 people in the worst tragedy to strike the annual Muslim pilgrimage in a quarter-century.

The disaster, which also injured several hundred people, was the second deadly accident to hit worshipers this month, after a crane collapse in the holy city of Mecca killed more than 100.

Iran's supreme leader said Saudi Arabia is responsible for Thursday's stampede during the annual hajj in which 90 Iranians were among more than 700 Muslim pilgrims killed.

Laying the blame for the deaths on "improper measures" and "mismanagement," Ayatollah Ali Khamenei announced three days of mourning.

The United States expressed alarm on Thursday over the case of a Saudi Arabian who was sentenced to death for his alleged role in anti-government protests as a teenager.

Saudi Arabia is a close ally of the United States, but State Department spokesman John Kirby said Washington was "deeply concerned" about the case of Ali Mohammed al-Nimr.

The worst disaster in 25 years at the annual hajj pilgrimage has left critics questioning the Saudi government's attention to safety, despite billions of dollars invested in improving conditions.

A stampede on Thursday killed more than 700 people during a stoning ritual attended by hundreds of thousands of Muslim pilgrims from around the world.

A Lebanese pilgrim was reported missing Thursday following one of the worst-ever stampede tragedies at the annual Muslim pilgrimage in Saudi Arabia, which left at least 717 people dead and 863 others injured.

“We were told about four missing pilgrims of whom three have been found unharmed, and search is still underway to locate a fourth pilgrim,” Lebanon's general consul in Jeddah Ziad Atallah announced.

A stampede killed at least 717 people and injured hundreds more at the hajj in Saudi Arabia on Thursday, in one of the worst-ever tragedies at the annual Muslim pilgrimage.

It was the second deadly accident to hit the pilgrims this month, after a crane collapse in Mecca killed more than 100.

The father of a Saudi youth facing execution for taking part in pro-reform protests appealed to King Salman on Wednesday to spare his life.

The sentence against Ali al-Nimr, only 17 when he was arrested in February 2012, has drawn international condemnation over his young age at the time and allegations that he was tortured into making a confession.

A Saudi Arabian detainee said by the U.S. military to have been a bodyguard for Osama bin Laden has been repatriated from Guantanamo Bay after more than a decade behind bars, the Pentagon said Tuesday.

Abdul Shalabi, 39, was first captured by Pakistani forces in December 2001 and was transferred to Guantanamo the following month.

Saudi Arabia must not execute a man who was convicted of joining a criminal group after participating in Arab Spring protests when he was 17, U.N. rights experts said Tuesday. 

Ali Mohammed al-Nimr was in high school in 2012 when he joined reform protests in Qatif in the eastern part of the kingdom. 

Hundreds of thousands of Muslims from across the globe began the annual hajj pilgrimage on Tuesday in one of the largest annual gatherings of people in the world.

In what for many will be the highlight of their spiritual lives, pilgrims began moving from Saudi Arabia's holy city of Mecca to nearby Mina for the start of the six-day event.