Kenya pledged to reopen its main airport to all international flights on Thursday night in a bid to reassure travellers that east Africa's transport hub could recover quickly from a fire that left the arrivals terminal a smouldering, blackened shell.

Jomo Kenyatta International Airport, sub-Saharan Africa's fourth busiest airport, was forced to close on Wednesday after a fire swept through the international arrivals building early in the morning.

On Thursday morning, the airport was only being used for domestic flights at a different terminal and limited international flights by Kenya Airways. Emirates , British Airways, Etihad, South African Airways, Ethiopian airlines and Rwanda Air were all forced to cancel their flights for a second day, tour operators said.

"The airport will be open for all other flights as from midnight on Thursday," said Michael Kamau, cabinet secretary for transport. He said a makeshift terminal would be built within days that could handle 1.5 million passengers.

White tents erected outside the domestic flights terminal were trying to cope with international travellers in the meantime.

In addition to the international arrivals building, Reuters TV saw considerable damage to the international departures terminal 50 metres away. The tunnel that connects the international arrivals and departures section was obstructed by a partially collapsed roof and sodden flooring.

"What is key is the speed at which this is normalised. If they can pull a rabbit out of the hat and get international flights moving they may bail themselves out," said Aly Khan Satchu, a Nairobi-based analyst.

"If this drags on it's going to damage Nairobi's regional hub status."

Investigators said it was too early to ascertain the cause of the fire but had ruled out terrorism. Wednesday marked the 15th anniversary of an attack by Islamist militants on the U.S. embassies in Nairobi and Dar es Salaam, the commercial capital of neighbouring Tanzania.

Kenyan media on Thursday criticised emergency responders for being too slow. Newspaper photos showed soldiers fetching water to put out the fire using buckets.

One Kenya Airports Authority official told Reuters the fire had put pressure on builders to complete construction of a new international arrivals terminal.

"We might just have to fast-track the new terminal. But there's still a lot to be done internally. The new unit won't be ready for another year," he said, asking not to be named because he was not authorised to talk to the media.

The airport, which is already operating at more than twice its 2.5 million passenger capacity, was expected to handle around 15 million passengers once the new terminal opened, the official said.

The fire is a blow to Kenya at the start of the peak tourism season. The horticulture industry, another major foreign exchange earner for east Africa's biggest economy, was also preparing for losses.

"We still haven't flown any produce out. We are waiting to see what rolls out today," Jane Ngige, chief executive officer of exporters association Kenya Flower Council, told Reuters.

The government's concerns about its image were reflected in the comments of some passengers who said they would now travel alternative routes.

"I think I'll skip this route for a wee while," said 39-year-old Terence Forster, a Tanzania-based Briton whose flight home was delayed.

"It's been my main transit route for a while. But not anymore. I can't see anything happening soon to get this airport up and running normally," he said. - Reuters