Four people have been killed when a helicopter exploded into a fireball after reportedly clipping the world's longest zip line on Saturday.

Shocking footage shows the doomed chopper spinning out of control after it's said to have hit the Ras al-Khaimah wire near Dubai.

Officials confirmed the Agusta 139 rescue helicopter was speeding towards an emergency before the horrific accident.

Horrified eyewitnesses told how it spun into a death spiral after the rotors got tangled in the steel wire, Gulf News reports.

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Onlooker Salam Naif said: "The chopper went into a spin all of a sudden and then crashed into the mountain. It burst into a ball of fire."

The National Search and Rescue Centre confirmed that four crew members were on board when the helicopter crashed.

The victims were named as pilots Humaid Al Zaabi, Saqr Al Yamahi and Jassim Al Tunaiji, along with South African paramedic Mark T. Al Zaabi.

Ras al-Khaimah ruler Sheikh Saud bin Saqr Al Qasimi last night ordered an immediate investigation into the crash.

A senior aviation official told Gulf News: "The accident happened at 5:50pm on Saturday and the helicopter was on a mission to airlift an injured man from Jebel Jais.

"The helicopter crashed before reaching the man."

The zipline opened last February as part of a tourist drive in the tiny desert emirate.

This story originally appeared in The Sun.