Johannesburg (AFP) - A South African court has sentenced two members of a right-wing group convicted of high treason for plotting to kill President Jacob Zuma to eight years in prison, state prosecutors said Thursday.

The men were arrested in December 2012 on suspicion of targeting top African National Congress leaders including Zuma during the party's conference.

National Prosecuting Authority spokesman Nathi Mncube said the High Court on Thursday convicted Johan Prinsloo of "high treason and possession of ammunition" and jailed him for eight years.

His co-accused Mark Trollip had last year pleaded guilty to a charge of conspiracy and was also handed an eight-year jail term.

A third defendant, Martin Keevy, was found mentally unfit to stand trial and charges against him were withdrawn.

The men arrested in the central Free State province were members of the Federal Freedom Party (FFP), which wants a separate state for white minority Afrikaners.

Charges against the fourth suspect, Hein Boonzaaier, then the party leader, were dropped due to insufficient evidence, according to Mncube.