At least 450 people have been killed and 700 more injured in a stampede near the Islamic holy city of Mecca, Saudi officials say.

The incident occurred as millions of Muslim pilgrims converged on Mina, a large valley about 5km (3 miles) from Mecca, for a tradition to throw stones at three pillars representing the devil, the last major rite of the Hajj or pilgrimage.

Saudi officials said that 220 emergency and rescue units and 4,000 personnel had been sent to the scene, with the injured taken to four local hospitals.

The Saudi civil defence directorate estimated that the victims of the stampede were from a variety of “different nationalities”, but no further details were given.

It is currently unclear what caused the stampede.

In a statement, Prime Minister David Cameron expressed his sympathies for the families of those affected:

“My thoughts and prayers are with the families of those killed in a stampede at the annual Hajj pilgrimage near Mecca.”

The incident is the second tragedy of this year’s Hajj after a crane collapsed at Mecca’s Grand Mosque earlier this month, killing 109 people.