Aviation staff at Kenya's main international airport will go on strike from next week over a contested investment proposal from an Indian conglomerate, their union said on Monday.

About 10,000 members of the Kenya Aviation Workers Union will walk out from August 19, its secretary general Moss Ndiema told AFP.

The dispute centres on plans by the Kenyan government to enter into an investment agreement with India's Adani Group to expand, modernise and operate Nairobi's Jomo Kenyatta International Airport (JKIA).

"We are opposed to the Adani deal which we think has not followed the due process of the law," said Ndiema.

Under the proposal, Adani would invest $1.85 billion to expand the airport in exchange for operating it for 30 years, according to documents seen by AFP.

Adani would add a second runway and upgrade the passenger terminal, according to the airport operator Kenya Airport Authority (KAA).

"We suspect rightly that this public private partnership idea is laced with corrupt intents," said Ndiema.

"People want to rip off KAA, in short, they want to take away JKIA and privatise it without following the due process."

- Airport 'made of canvas' -

Kenya's government has defended the deal as a necessary measure to refurbish JKIA, one of Africa's busiest hubs, often hit by power outages leaking roofs.

"The airport we have in Nairobi is made of canvas," President William Ruto said last month. "We need to work with investors to give us a new airport."

"I have seen very many people saying William Ruto wants to sell the airport, am I a madman? How do you sell a strategic national asset? You have to be insane," he told a town hall meeting in the coastal city of Mombasa.

KAA said last month that the deal would be "subjected to technical, financial and legal reviews alongside requisite due processes", without giving a timeline.

Last year, Adani Group, the sprawling ports-to-power Indian conglomerate, saw billions of dollars wiped from its market value after a report by the US-based Hindenburg Research firm accused it of "brazen" corporate fraud.

Gautam Adani, the family-run conglomerate's founder and the world's 12th richest person according to a Bloomberg index of the world's billionaires, denied Hindenburg's allegations and called its report a "deliberate attempt" to damage its image for the benefit of short-sellers.

According to the KAA, 8.8 million passengers and 380,000 tonnes of cargo passed through JKIA in the 2022/23 financial year.

The airport contributes just over five percent of Kenya's gross domestic product.