A judge overseeing the corruption trial of former South African President Jacob Zuma has stepped down, causing potential delays.

Judge Piet Koen cited concerns over compromising the perception of justice after denying Zuma’s efforts to remove the prosecutor.

Zuma faces 16 charges of fraud and racketeering related to the 1990s purchase of fighter jets and other arms equipment.

He was previously sentenced to 15 months in prison for contempt of court in a different corruption case.

Zuma denies any wrongdoing.