Kenya's defence chief and nine other top brass died on Thursday in a military helicopter crash in a remote area of the country, President William Ruto said.

"Today at 2:20 pm, our nation suffered a tragic air accident... I am deeply saddened to announce the passing of General Francis Omondi Ogolla, the Chief of the Kenya Defence Forces (CDF)," Ruto told reporters.

The president, who had convened an urgent meeting of the National Security Council after news of the accident emerged, said nine other "gallant military personnel" on board were also killed while two survived.

Ogolla, 61, a trained fighter pilot, had only been in the post for a year but was shortly due to mark 40 years of military service.

Ruto said the Kenya Air Force had dispatched an investigation team to establish the cause of the crash, which took place in Elgeyo Marakwet county, about 400 kilometres (250 miles) northwest of the capital Nairobi.

Ogolla's helicopter went down shortly after takeoff from the village of Chesegon, where he and his entourage had been visiting a school after making stops in other areas to see Kenyan troops and other sites.

- 'Distinguished four-star general' -

Ruto announced three days of mourning from Friday, with official flags to fly at half-mast, while messages of condolence were sent from across the region.

"A distinguished four-star general has fallen in the course of duty and service of the country," Ruto said.

"Our motherland has lost one of her most valiant generals, gallant officers, service men and women."

The bodies of the victims, draped in Kenyan flags, were returned to Nairobi on an air force plane late Thursday, according to images shared by the presidency.

Kenyan media reports said this was the fifth military chopper crash in 12 months, with claims the aircraft were old and poorly maintained.

In June 2021, at least 10 soldiers were killed when their helicopter crashed during a training exercise south of Nairobi.

Ruto said Ogolla had left Nairobi on Thursday morning on a "Huey" helicopter to visit troops in the North Rift area deployed as part of Operation Maliza Uhalifu (Operation End Crime in Swahili).

Kenyan authorities have long battled insecurity in the North Rift region, which is marred by armed bandits and cattle rustlers.

The Bell UH-1B helicopter, nicknamed the "Huey", was developed in the 1950s and widely used by the US military during the Vietnam War.

- 'Significant loss to the nation' -

Ogolla, a married father of two, was appointed Chief of the Defence Forces by Ruto in April last year, a position that also serves as the top military adviser to the president.

Under Kenyan military regulations, the defence chief normally retires at 62 or after four years in the post, whichever comes first.

Ruto told journalists last May that he appointed Ogolla despite him being among those who tried to overturn his narrow election win against opposition leader Raila Odinga in 2022.

"When I looked at his CV, he was the best person to be (a) general," Ruto said.

Ogolla joined the KDF in April 1984, rising through the ranks to command the Kenyan Air Force in 2018, a post he held for three years before becoming vice chief of the defence forces in 2021.

Ruto's predecessor Uhuru Kenyatta said Ogolla's death was a "significant loss to the nation".

"The CDF was not only an accomplished military leader, but also a devoted patriot who dedicated his life to serving and protecting our beloved country."

Messages of condolence were also sent by, among others, the African Union, regional grouping IGAD and the US, British and EU embassies in Kenya.

"This loss is not only felt by Kenya but also by the entire region," IGAD secretary general Workneh Gebeyehu said on X.