Guinea's military chief, General Kelefa Diallo, was on the plane on a security mission to Liberia, police said.
Mr Diallo was a close ally of Guinean President Alpha Conde.
Security officials in Liberia said at least 10 people were killed in the crash, which occurred at Charlesville, some 25 miles south east of Monrovia.
"Senior Guinean military and defence officials were on the plane," Liberian Defence Minister Brownie Samukai said. "An investigation in now under way into why it happened."
Liberian President Ellen Johnson-Sirleaf declared today a national day of mourning.