Dramatic plumes of black smoke billowed out of the main arrivals terminal, but by 9:00 am (0600 GMT), some four hours after the blaze broke out, 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.
Some 16,000 passengers usually transit through the airport every day, Esipisu added.
The airport -- which hosts both international and domestic flights -- serves as a regional hub for east Africa, with many long-distance international flights landing there to connect to countries across the region.
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Chunks of the roof of the international arrivals terminal had caved in.
Tens of thousands of passengers will be affected by the closure of the airport, while regional airports have limited capacity and will likely struggle to handle all the travellers unable to land in Nairobi.
The fire was reported to have begun at the immigration section at arrivals, but the cause is not yet known.
Police chief David Kimaiyo said that investigations had begun, as he urged the public to remain calm.
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".
However, the runway is not affected and cargo flights and aircraft whose passengers were already checked in were allowed to take off.