NAIROBI (AFP) - A devastating fire shut down Nairobi's international airport for much of Wednesday in a "major crisis" that forced the cancellation or diversion of scores of flights at east Africa's biggest transport hub.

The first international flights by Kenya's national carrier will land early Thursday in Nairobi, officials said, with immigration formalities and baggage processed through what is normally the domestic terminal as international arrivals were gutted in the four-hour blaze.

Cargo and domestic flights out of Nairobi resumed early Wednesday evening.

It was not clear whether other airlines would also land as early as Thursday.

The inferno, which started before dawn, sent dramatic plumes of black smoke billowing out of the main arrivals terminal.

But by 9am (2pm Singapore time) firefighters had succeeded in stemming the raging flames, despite being hampered by a lack of water and equipment.

"Everything is being done to resume normal operations," said presidential spokesman Manoah Esipisu, speaking to reporters outside the gutted arrivals hall of the Jomo Kenyatta International Airport (JKIA), where flames had earlier lit up the pre-dawn sky.

International passengers were given priority to fly to Kenya's second city Mombasa to connect to onward flights from there, while Nairobi's domestic terminal was being prepared to handle international flights starting Thursday.

Some 16,000 passengers usually transit through JKIA every day, Esipisu said.

The airport is a regional hub for east Africa, with many long-distance international flights landing there to connect to countries across the region.

Kenyan business operators were eager for a swift return to normal operations following the disaster.

"This is the busiest time of year for international flights," said Duncan Muriuki, head of Destination Africa, a tour operator.

"We have people stuck in Mombasa, so it will definitely impact us negatively," he said, adding that clients blocked at the coast would face journeys of several hours to travel by road to the capital.

The flower sector, which provides a livelihood for some half a million Kenyans, could be hard hit by the fire, as any delay in shipping out fragile lilies and roses would result in big losses.

Jane Ngige, who heads the Kenya Flower Council, said harvesting at flower farms had been halted on Wednesday while the trade association waited for more information.

Tens of thousands of passengers could be affected by the JKIA's closure, while regional airports have limited capacity and will likely struggle to handle all the travellers unable to land in Nairobi.

Around 250 flights take off and land there every day, regional aviation sources said, adding that some six million passengers used the airport last year.

The fire was reported to have begun at the immigration section at arrivals, but the cause is not yet known, with police chief David Kimaiyo saying that investigations had begun.

No casualties have been reported, but two people - an airport worker and a passenger - were taken to hospital for smoke inhalation, Kenya Airways' chief Titus Naikuni said.

The interior ministry was forced to issue public appeals for Nairobi's notoriously congested traffic to give way to trucks ferrying water to the airport after firefighters tackling the blaze ran "dangerously low on water".

Soldiers and police even came with buckets to help put out the fire, said Sylvia Amondi, who was at the airport to pick up a relative who had been due to arrive there.

"The international arrivals station has been completely destroyed, the roof has caved in and the floor is covered in debris and water," she said.

Behind the scenes portly civil servants braved massive puddles to sift through soggy papers and files in what was left of their offices.

In the baggage claim area only a giant advert for multi-choice TV remained untouched.

President Uhuru Kenyatta, whose father Jomo Kenyatta - Kenya's first president - the airport is named after, visited the charred terminal along with his top security chiefs to assess the damage caused by the "huge inferno", the interior ministry added.

Mr Kenyatta later called the fire a "tragic event", and promised all efforts would be made to resume normal operations "as soon as possible".

August is one of Kenya's busiest months for tourism, a key industry for the country, as foreign travellers fly in to see its wildlife and enjoy the white sand beaches on its Indian Ocean coast.

The airport offers direct connections to Europe, the Middle East, the Far East and other African cities.