A small plane carrying medical staff crashed in a densely-populated neighbourhood in Kenya's capital on Thursday afternoon, killing six people, local authorities said.

The Cessna plane, run by AMREF Flying Doctors, came down as it was flying to Hargeisa, the capital of Somalia's breakaway territory of Somaliland, the charity said in a statement.

Kiambu County Commissioner Henry Wafula said the four people on board and two people on the ground were killed.

The Kenya Civil Aviation Authority said the plane took off from Wilson Airport at 02:14 PM (1114 GMT) and lost contact with air traffic control three minutes later.

"We are deeply saddened by this tragic incident involving an air ambulance mission," KCAA said in a statement.

One witness, who asked not to be named, told Reuters she heard a loud sound, then saw a fireball and debris scattered over the neighbourhood.

Unverified videos shared on social media showed firefighters trying to douse burning debris inside what appeared to be a residential compound.

Crashes involving light aircraft are relatively common in eastern Africa.

In April last year, Kenya's military chief was among 10 people killed when their military helicopter crashed shortly after take-off.