A helicopter crashed in central London and plunged to the ground in a ball of flames on Wednesday, with police saying it had hit a crane on top of a building.

Police so far said two people have been confirmed dead but did not say if the deaths were passengers and crew in the helicopter or people on the ground at the scene of the crash, an under-construction apartment block in Vauxhall on the banks of the River Thames.

Fire engines and police were at the scene of the crash near the River Thames at Vauxhall, where a huge plume of smoke billowed into the sky.

Paul Ferguson, who was working in an office near the incident, told BBC News: "There was a flash and the helicopter plunged to the ground. It exploded and you can imagine the smoke coming out of it."

Witnesses posted photographs of burning wreckage on social networking sites.

London's Metropolitan Police said the helicopter was not one of its aircraft. "We had a call at approximately 8am this morning and we are dealing with an incident," a police spokesman said.

The crash was causing long tailbacks on roads during the morning rush hour.

More to come.