The internal memo, marked as "secret," was sent to CNN and has been verified by a high-level source in the Kenyan aviation industry who received the note late last week.
The memo warns airport employees that al Qaeda-linked terror group Al Shabaab would deploy suicide bombers trained in Somalia
"on airborne suicide missions."
However, since the memo was leaked to local media, Kenya's Aviation Authority has released a statement from its managing director, Yatich Kangugo, saying the earlier memo had been released before the information had been vetted by relevant authorities.
"We therefore wish to assure our citizens that all airport users that KAA airports and airstrips are not under any imminent threat. I wish to report that normal operations are ongoing." Kangugo also said the operational threat category had been raised to high alert "in conjunction with all other national security organs who have increased vigilance to counter any potential threats."
Security has been increased at both Nairobi's major airport, Jomo Kenyatta International, and Mombasa's major airport, Moi International.
CNN's source at the airport has said that most of the attention from security officials has been focused on Terminal 2 at Nairobi's airport, which is the domestic terminal and where most planes bound for Somalia are flown out of and into.
A fire severely damaged Nairobi's international airport in 2013
, and many of the terminals have since been under reconstruction. Some, including parts of Terminal 2, consist of temporary structures.
The threat warning comes less than a month after a suspected terrorist carried a bomb disguised as a laptop onto a plane in Somalia's capital, Mogadishu. The bomb blew a hole in the fuselage of the plane, and the suspect was sucked out of the hole and plunged to the ground. The explosion killed Abdisalam Abdullahi Borleh
and injured two other passengers.
The pilot of the Daallo Airlines flight was able to land the plane safely. At the time, experts warned that the bomb was "sophisticated"
and that it was disguised well enough as an electronic device to fool airline security officials manning the X-ray machines at Mogadishu international Airport.
Authorities in Somalia are still investigating who else was involved in the elaborate plot, but Al Shabaab has since claimed responsibility.
Al Shabaab has conducted several terror attacks inside Kenya since the country sent its troops across the border to Somalia to battle the terror group in 2011. These attacks include the Westgate mall attack in September 2013
that killed more than 60 people and the Garissa University attack in 2015
, where nearly 150 people died, many of them university students.
The group just days ago released a propaganda video claiming to have fighters inside Kenya who are ready to attack.