Six people have been killed after a light aircraft belonging to a medical charity crashed in Kenya's capital, Nairobi, according to a local official.

Charity Amref Flying Doctors said the Cessna plane took off from Wilson airport on Thursday afternoon and was en route to Hargeisa in Somalia when it crashed and burst into flames at a residential building in Nairobi's Githurai area.

Kiambu County Commissioner Henry Wafula said four people on the plane were killed, including doctors, nurses and the pilot - as well as another two people on the ground, while two others were seriously injured.

Investigators have been despatched to the scene of the crash to establish its cause.

The plane lost both radio and radar contact with air traffic control just three minutes after take off, the Kenya Civil Aviation Authority said.

There were four crew and Amref staff on board, the charity said.

"At this time, we are cooperating fully with relevant aviation authorities and emergency response teams to establish the facts surrounding the situation," Amref CEO Stephen Gitau said in a statement.

The Kenya Defence Forces and the National Police Service have been deployed to the scene to conduct search and recovery operations.

Patricia Kombo, an eyewitness, told the BBC that she was in a cab with her friends heading to Githurai when they heard a loud bang and a red flash ahead of them.

"Before I could take my phone to record the flash was gone and smoke was billowing. We then heard people screaming and running and so we ended our trip.

"We then discovered it was a plane crash and saw the sunken hole the crash had created in the ground," she said.

In a separate incident, a train and a bus collided at a railway crossing near Naivasha town, central Kenya, killing at least four people, according to Reuters news agency citing a Red Cross worker.

The Kenya Pipeline Company, whose bus was involved in the incident, said it was carrying staff finishing their morning shift at one of its training centres and that all injured staff had been taken to hospital for treatment.