ISTANBUL, Nov. 26 (Xinhua) -- At least four soldiers were killed and another injured in critical condition on Monday when a military helicopter crashed in a residential area in Istanbul.

The helicopter fell on a street filled with houses in the Sancaktepe district on the Asian part of the city, a video released by the NTV broadcaster showed.

Istanbul Governor Ali Yerlikaya said in a written statement that "the military helicopter crashed during a training mission for unknown reasons."

He said an investigation into the accident has started.

NTV claimed that the helicopter crashed after hitting the roof of a building while trying to land on the street due to a malfunction.