By Alphonso Toweh and Saliou Samb

MONROVIA/CONAKRY (Reuters) - The head of Guinea's armed forces and other senior military officials from the West African state were killed on Monday when their aircraft crashed close to the Liberian capital Monrovia, a Guinean presidential official said.

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

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

Guinea's military chief, General Kelefa Diallo, was on the plane on a security mission to Liberia, police in Guinea said.

Diallo was a close ally of Guinean President Alpha Conde, who was elected to lead the world's top bauxite producer in late 2010, ending two years of military rule.

Security officials in Liberia said that at least 10 people were killed in the crash, which occurred at Charlesville, some 40 km (25 miles) southeast of Monrovia.

"Senior Guinean military and defense officials were on the plane," Liberian Defense Minister Brownie Samukai told Reuters. "An investigation in now underway into why it happened."

Liberian President Ellen Johnson-Sirleaf declared a national day of mourning for Tuesday.

General Diallo was one of the major architects of reform of Guinea's restive military, which had seized power in the former French colony in 2008. Some 4,000 soldiers were forced to retire under a U.N.-backed scheme to slim the bloated armed forces.

Diallo's predecessor, Nouhou Thiam, is in prison facing trial for his alleged role in a gun and rocket attack on President Conde's home by soldiers in 2011.

Conde's government has been trying to organize legislative elections for May, the final step in the transition back to civilian rule and a prerequisite to unlock millions of dollars of frozen foreign aid.

The opposition, alleging bias in the electoral authority, has called protests for Wednesday this week. Conde's 2010 election in a vote hailed as the first free elections since the end of French rule in 1960 was marred by deadly riots and opposition allegations of fraud.

(Writing by Daniel Flynn; Editing by Pascal Fletcher)