Emergency workers rushed to the scene of a helicopter crash Wednesday south of the River Thames in central London, authorities said.
The helicopter appears to have hit a crane on top of a building on Wandsworth Road, London's Metropolitan Police said.
"It is too early at this stage to confirm any casualties," a police statement said
London's Metropolitan Police said they were called to the incident at 8 a.m., when many commuters would be traveling through the area.
Pictures from the scene show billowing smoke and flames.
Many emergency vehicles were on the scene and roads in the area were cordoned off.
CNN producer James Whipps, who was in an office in the area, heard the sound of the helicopter rotors suddenly cut out.
He did not see the impact but as he looked though a window, he "suddenly saw this enormous fireball, black smoke, shoot up from the side of a building."
He said the helicopter wreckage appeared to have landed on two cars.
The crane on top of the tall building is badly bent from the impact, he said.
The emergency services were very quick to get to the scene, and the main fire was swiftly extinguished, he said.
Many helicopters follow the Thames as a flight route through the city. This helicopter appeared to be flying further south than usual, he said.
Vauxhall, not far from Westminster, is where a new U.S. Embassy is being built.
The area, which is a mix of industrial, business and residential use, is also home to the headquarters of the UK intelligence service, MI6.
Visibility was poor in London Wednesday morning.