At least 310 people have died after a stampede on the outskirts of the holy city of Mecca during the annual hajj pilgrimage.
Although it is not clear if any British nationals are caught up in the devastation in Mina, the Foreign Office has said it was in contact with authorities.
At least 450 other pilgrims have been injured in the crush as Muslims around the world marked Eid al-Adha.
A Foreign Office spokesman said: "We are in contact with local authorities and urgently seeking more information following reports of a crush during the Hajj pilgrimage at Mecca."
Some two million people are taking part in this year's hajj pilgrimage, which began on Tuesday.
Muslims travel to the site in order to throw pebbles and stones at the walls in a symbolic act of stoning of the devil.
Amateur video and pictures on social media showed images of dead bodies on the ground dressed in the simple terry cloth garments worn during the hajj.
Other pictures show people sitting in wheelchairs and being treated.
It is not the first time there has been a crush as people push and shove their way forward to get close to the walls.
In 2006 more than 360 pilgrims were killed in a stampede in the same area.
Another stampede at Mina in 2004 left 244 pilgrims dead and hundreds injured.
The accident comes less than two weeks after a giant construction crane came crashing down on the Grand Mosque in the Saudi city of Mecca, the focal point of the hajj.
Father-of-four Qasim Akram, 32, from Bolton, Greater Manchester, was killed in the incident after arriving ahead of the start of the annual pilgrimage.
It was his first pilgrimage and had only arrived in the country the morning of the crash.
British nationals travel to the pilgrimage each year after saving for the "once in a lifetime trip".
It is estimated that around 25,000 British nationals head overseas from the UK to Saudi Arabia for the hajj, according to Abta, the travel association.
Each year pilgrims pay hundreds of thousands of pounds to go on the religious trip, with people spending between £4,000-5,000 for a typical pilgrimage.
This year's hajj takes place between September 21-6.