At least two people were killed and several others injured after a helicopter hit a crane that was on top of a high-rise building under construction in Vauxhall in central London and exploded on the ground Wednesday morning.

According to London's Metropolitan Police, two people have been confirmed dead at the scene. One person was taken to a south London hospital in critical condition. Five people were taken to hospitals with minor injuries and four were treated at the scene.

The police said the cause of the crash was not known.  There was nothing that suggested a terrorism link to the incident, they said.

The BBC reported that one of the dead was in the helicopter.

Witnesses told CNN that they heard a bang and saw an “enormous fireball, black smoke, shoot up from the side of a building."

"There was a flash and the helicopter plunged to the ground. It exploded and you can imagine the smoke coming out of it," Paul Ferguson, an office worker near the scene, told the BBC.

Emergency service vehicles were on the site and security personnel had cordoned off the area. Wandsworth Road was closed and traffic in nearby areas had come to a standstill, local media reported.

The crane, badly bent due to the crash impact, was seen dangling from the roof of the building, CNN reported.

The exact reason for the crash was not known, but local media reported that visibility was very poor in London during the morning hours.