* Busiest airport in east Africa closed after fire
* Arrivals building devastated, flights diverted
* No casualties reported
By Drazen Jorgic
NAIROBI, Aug 7 (Reuters) - A fire engulfed Kenya's main airport on Wednesday, forcing the indefinite suspension of international passenger flights and choking a vital travel gateway to east Africa.
The blaze lit up the early morning sky, the billowing clouds of black smoke engulfing the terminal buildings visible from miles away.
The intense heat repeatedly drove back firefighters who battled for five hours to put out the fire, the worst on record at Jomo Kenyatta International Airport, east Africa's busiest.
Michael Kamau, cabinet secretary for transport said an investigation into the fire would start immediately but that it was too early to speculate on the cause.
The blaze stranded thousands of passengers at the airport and exporters of perishable produce, mainly flowers, feared for their export-driven business, a leading source of foreign exchange for Kenya alongside tea exports and tourism.
"This is disastrous," Jane Ngige, chief executive officer of exporters association Kenya Flower Council, told Reuters.
There were no immediate reports of casualties from the fire, which started in the arrivals and immigration area. Business travellers and tourists were diverted to other airports in the region.
Shares in Kenya Airways, which uses the airport as its main hub, fell 2 percent. Foreign carriers using the terminal include Emirates, British Airways, Qatar, KLM ), Turkish Airways, South African Airlines and Ethiopian airlines.
Kenyan authorities said domestic flights and international cargo flights would resume later on Wednesday. Plans for international flights would be announced later.
"Right now we've allowed the cargo to come in. Fruits, flower are coming in and are being processed. We hope we will be able to resume domestic flights," Kamau said.
Inside the gutted building, neat lines of metal trolleys with melted plastic handles were the only clear reminder that the building - whose roof partially collapsed - was once an airport terminal.
Some passengers searched for their luggage amid the charred ruins while outside, staff from Western embassies waved their national flags to attract passengers looking for a route home or a place to stay.
The fire coincided with the 15th anniversary of a twin attacks by Islamist militants on the United States embassy in Nairobi and Dar es Salaam, the commercial capital of neighbouring Tanzania.
But security analysts said there was as yet no indication of any link to Islamist militants that Kenyan soldiers are battling in neighbouring Somalia as part of an African Union force.
"It doesn't bear the hallmarks of an al Shabaab operation but one never knows. It might be something new," said a regional security analyst, speaking on condition of anonymity.
The fire was a major blow to Kenya right at the start of the busiest period of the tourism season, a key sector for the Kenyan economy and an important source of foreign currency earnings.
Tourists to the world famous Masai Mara Game Reserve had cancelled their bookings, tour operators said.
Flights were diverted to Kenya's port city of Mombasa. Plans were underway to divert other flights to Eldoret in the northwest and Kisumu in the west, as well as to neighbouring countries including Uganda, Tanzania and Rwanda.
Passengers faced bus trips of hundreds of miles to reach the Kenyan capital.
"I had two appointments today in Nairobi which I have missed because of this," Susan Eklow, 42, from Sweden told Reuters in Mombasa.
"We haven't been told anything except to hang around here as a solution was sought," said Muses Heuwaggen, 45, a German tourist in Mombasa headed to Naivasha in the scenic Rift Valley.