"No KQ (Kenya Airways) aircraft has departed Jomo Kenyatta International Airport flown by a KALPA (Kenya Airline Pilots Association) member from 6 a.m. this morning," the union said in a statement.
"The strike is fully in force."
The Kenya Airline Pilots Association (KALPA) said the carrier's management declined to listen to their proposals on how resolve their grievances.
The airline's managing director and CEO, Allan Kilavuka, said 23 flights had been cancelled as of 11:00 am due to "the unlawful strike," affecting over 9,000 passengers.
He urged the protesting pilots, who make up 10 percent of the workforce, to return to work by 10:30 am on Sunday.
"Failure to do so will lead to immediate disciplinary action," he warned.
The Kenya Aviation Workers Union (KAWU) subsequently announced that ground staff would also strike from 2:00 pm onwards in a separate, long-running dispute with the Kenya Airports Authority (KAA) over salary increases.
Kenya Airways apologized to passengers in a statement and said it would do its best to minimize the inconvenience, which could include rebooking them onto alternative flights.
The airline, part owned by the government and Air France-KLM, is one of the biggest in Africa, connecting multiple countries to Europe and Asia, but it is facing turbulent times, including years of losses.
Kenya's newly appointed Transport Minister Kipchumba Murkomen told reporters that the pilots' strike was unwarranted and "akin to economic sabotage".
"I am not saying their concerns are not valid," he said, appealing to the "goodwill of the pilots to terminate" what he described as drastic action.
The pilots are pressing for the reinstatement of contributions to a provident fund and payment of all salaries stopped during the COVID-19 pandemic.
Some information for this report came from Reuters.