Republic of Congo's top prosecutor says a presidential candidate who formerly served as armed forces chief of staff is being investigated for allegedly making statements that threaten to state security.

Andre Oko Ngakala said on state television Friday that Gen. Jean Marie Michel Mokoko had not responded to a summons for questioning. Security forces earlier blocked off access to Mokoko's home in the capital, Brazzaville.

In recent weeks, a video has been circulating on social media that purportedly implicates Mokoko in planning a coup attempt. His campaign has denounced it as a fraud.

Mokoko is one of 10 candidates challenging President Denis Sassou N'Guesso in a vote scheduled for March 20. Sassou N'Guesso officially filed his candidacy on Friday, one day before the deadline.