Kenya's military chief, Gen Francis Omondi Ogolla, has died after a military helicopter crashed in the west of the country, the president said.
Ogolla - Kenya's highest ranking military officer - was in the helicopter alongside 11 other military personnel. Only two people survived.
In a news conference, President William Ruto said it was a "moment of great sadness" for the country.
He had earlier convened an urgent security council meeting.
The crash happened at 14:20 (11:20 GMT), Mr Ruto said. The Kenyan air force has dispatched an air investigation team to establish the cause.
The helicopter came down in Elgeyo Marakwet county, some 400km (250 miles) north-west of the capital Nairobi, shortly after take off, Mr Ruto said.
Ogolla was appointed in April of last year after serving as the commander of the air force and deputy chief of defence forces.
Mr Ruto described his chief military adviser as a gallant officer who had died in the line of duty.
"Our motherland has lost one of her most valiant generals, gallant officers, service men and women," Mr Ruto told the nation.
Kenya will observe a three-day period of mourning, commencing on Friday, with flags flying at half-mast.
Ogolla first joined the Kenya Defence Forces on 24 April 1984, according to the Kenyan defence ministry's website.
He was due to mark 40 years in the military next week.
He began his career as a 2nd lieutenant in the country's air force, where he trained as a fighter pilot with the US air force, the ministry says. In 2018, he became commander of the air force.
Nine others killed in the crash were named as Brig Swale Saidi, Col Duncan Keittany, Lt Col David Sawe, Maj George Benson Magondu, Capt Sora Mohamed, Capt Hillary Litali, Snr Sgt John Kinyua Mureithi, Sgt Cliphonce Omondi, and Sgt Rose Nyawira.
Moussa Faki Mahamat, chair of the African Union Commission, said its "thoughts and prayers" went out to Kenya's president, government and people following the "tragic helicopter accident".
The two survivors are in critical condition and undergoing treatment.
The officers had travelled to Kenya's North Rift region, which has been plagued by banditry.
They were on a mission to reopen some of the schools closed following bandit attacks. They had also visited military officers deployed to stabilise the region.
In June 2021 at least 10 soldiers were killed when their helicopter crashed while landing near the capital, Nairobi.