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," 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.

He said the Kenya Air Force has dispatched an air 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.

The helicopter crashed shortly after takeoff from Chesegon, where he and his entourage had been visiting a school, Ruto said.

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

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

Ruto announced three days of mourning from Friday, with official flags flying at half mast.

He said Ogolla, 61, had left Nairobi on Thursday morning on an air force Huey helicopter to visit troops deployed in the North Rift area in Operation Maliza Uhalifu (Operation End Crime in Swalihi), and other sites.

Kenyan authorities have long battled insecurity in the Rift Valley region, with armed bandits and cattle rustlers rampant.

"The helicopter burst into flames after crashing and it had more than 10 senior commanders on board including General Ogolla," a police officer had told AFP earlier.

"They were in the area on a security mission because there are KDF (Kenya Defence Forces) soldiers deployed in the region," he added.

Ogolla was appointed Chief of the Defence Forces by Ruto in April last year after a stint as deputy.

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, adding his decision went against the wishes of many people.

A trained fighter pilot, 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.