Kenya's internal security minister, who was closely involved in the fight against Somali militant group Al Shabaab, was killed when the police helicopter he was travelling in crashed into a forest on Sunday.

George Saitoti, his deputy Orwa Ojode, two pilots and two bodyguards were killed when the aircraft came down outside the capital Nairobi and burst into flames, the government said.

A prospective candidate in next March's presidential election, Saitoti was one of the most outspoken government politicians on the threat from Somali militants, often visiting scenes of al Shabaab's attacks and vowing to crush the group.      
Kenya sent troops across the Somali border last October in pursuit of Al Shabaab, drawing a series of retaliatory grenade attacks on its soil, killing several people.

Prime Minister Raila Odinga said investigations into the cause of the crash had started and that the cabinet would hold a special session on the incident.

"This is a terrible tragedy that has struck our country this morning. Nobody knows exactly the cause of this accident. That is why experts will carry out investigations," Odinga told reporters at the scene of the crash. "We will do everything possible to ensure we find we find out the cause of this accident, but for now it is just an accident."

Witnesses said they saw a helicopter swaying wildly in the air at about 8:30 am (0530 GMT) in foggy weather. "I heard an explosion and I rushed to the scene," said 25-year old Leonard Njoroge whose house is a stone's throw away from the crash site.     

"Others came and we tried to put out the fire using sand but it was too fierce. It was too late for the passengers. We didn't know at the time it was carrying prominent people."     

 Debris of the burnt-out helicopter were strewn in the forest in the Ngong area just outside of Nairobi where government officials and curious locals jostled to catch a glimpse.

Saitoti, who was a long-serving vice president under former President Daniel Arap Moi, planned to run for president for President Mwai Kibaki's Party of National Unity (PNU).

"He leaves behind a rich legacy of service to the country," Kibaki said in a statement of condolence. Other candidates in the presidential race, including Odinga and his deputy Musalia Mudavadi, cancelled their campaign rallies.   

The death of Saitot, a former mathematics professor who made a fortune in real estate, came exactly four years after another cabinet minister and an assistant minister were killed when a small plane they were travelling in crashed.

The death of Kipkalya Kones, minister for roads, and Lorna Laboso, an assistant minister in the office of the vice president, prompted the government to issue a directive against senior officials travelling in the same aircraft. - Reuters