* Busiest airport in east Africa closed by fire
* Arrivals building devastated
* No casualties reported; flights being diverted
By Drazen Jorgic
NAIROBI, Aug 7 (Reuters) - A fire engulfed Kenya's main airport on Wednesday forcing the closure of a vital travel and trade gateway to east Africa.
Flames from the inferno lit up the early morning sky, and the intense heat repeatedly drove back firefighters. A huge plume of black smoke billowed from the airport buildings, but there were no immediate reports of casualties or injuries.
Hundreds of passengers were stranded outside Jomo Kenyatta International Airport in the capital Nairobi, which was cordoned off to keep the public out after the fire started early in the morning in the arrivals and immigration area, officials said.
The cause of the worst fire in record at east Africa's busiest airport was not yet known.
"We are still fighting to contain the fire. Investigations will start immediately after," Michael Kamau, cabinet secretary for transport, told reporters at the airport.
"The fire started at a very central part of the airport and this made access difficult. But we have closed the airport indefinitely as we try to contain the fire."
One passenger at the airport said he heard two small explosions from the international arrival area, then sirens.
"I was waiting for my flight around 5 a.m. (0200 GMT) when I heard two explosions, as if from gas cylinder or electricity fault," said the passenger, a Kenyan who requested anonymity.
"Within minutes, the entire airport was secured by police and Kenya airport authority personnel, who ordered everybody out of the airport," he said.
The airport buildings were ravaged by the fire. Television pictures showed desks that had been reduced to chacoal inside the burned out terminal. The roof had partly caved in, and the floor was flooded with water from fire fighters.
Transport secretary Kamau said no plane had landed or departed since the fire started, and said he could not give any indication of when the airport would reopen. Airlines that wanted to take off without passengers have been allowed to do so, he said.
TRAVELLERS STRANDED, EXPORTERS WORRIED
Stranded passengers watched from a short distance as grey smoke continued to billow from the blackened building. Many sat on their bags or on the ground as they waited for information.
Svein Huseby, 38, an engineer from Norway had been due to fly home after visiting his Kenyan girlfriend.
"It's been chaotic ... No one knows if they will be going home in two hours, two days or two months. People are going crazy," he said.
"Some official information would have been nice."
Exporters of perishable produce, mainly flowers, said the impact of the fire would cripple their business.
"This is disastrous," Jane Ngige, chief executive officer of exporters association Kenya Flower Council, told Reuters.
Airlines are expected to divert flights to Kenya's port city of Mombasa, Eldoret in the northwest and Kisumu in the west, as well as to neighbouring countries including Uganda, Tanzania and Rwanda.
Cabinet Secretary for the Interior Joseph ole Lenku said security had been heightened at the airport after the fire started.
"People should not come to the airport while this work is ongoing. No casualties have been reported and the fire fighters are doing a good job. Safety is paramount," he said.
Kenya Airways, one of the leading airlines in the region which uses the airport as its main hub, said it was diverting flights to Mombasa and that transit passengers were being taken to hotels. Other flights had been cancelled.
Shares in the airline fell 2 percent.
In Rwanda, Robert Nsinga, a spokesman for RwandAir said it had cancelled flights to Nairobi and was offering passengers flights to Mombasa instead.
Wenceslaus Rama Makuza, the chief executive officer of Uganda's Civil Aviation Authority, said no international flights had been diverted to its main airport yet, but one Air Uganda flight had been turned away from Nairobi due to the fire.
Two days ago, a fuel pump failure caused big delays at Nairobi airport.
It was also in the news last week when several duty free shops were forcefully re-possessed by the Kenya Airports Authority (KAA) in the early hours of Thursday, following the expiry of a lease agreement.