"There was the chief of staff and five other military officials on the plane," said the source in Conakry, who asked not to be identified.

"It's clear that everyone on the plane is dead."

The Guinean military chief, General Kelefa Diallo, was on a security mission to Liberia, police in Guinea said. Diallo was a close ally of Guinean President Alpha Conde.

Security officials in Liberia said earlier that at least four people were killed in the crash, which occurred at Charlesville, some 40 km (25 miles) southeast of Monrovia.

"Senior Guinean military and defence officials were on the plane," Liberian Defence Minister Brownie Samukai told Reuters.

"An investigation in now underway into why it happened."