SEOUL (AFP) - The South Korean man who slashed the face of the US Ambassador to Seoul in a knife attack in March has been additionally charged with promoting North Korea, a report said Wednesday.

Kim Ki-Jong, 56, already stands charged with attempted murder for his assault on Mark Lippert at a breakfast function in Seoul that left the ambassador needing 80 stitches to a deep gash on his cheek.

The new charge, laid out during a hearing at the Seoul Central District Court on Wednesday, is related to South Korea's strict National Security Law that bans any act seen as aiding North Korea or promoting its ideology.

Convictions can bring jail terms of up to seven years.

"The accused promoted North Korea's allegations that joint South Korea-US military drills are a rehearsal for an aggression against the North," Yonhap news agency quoted a prosecutor as saying at the hearing. "In order to stop the exercise, he assaulted the ambassador," he said.

Books and other materials confiscated from Kim's home were also suggestive of his support for North Korean causes, the prosecutor added.

Kim is a known maverick activist who had been handed a two-year suspended sentence in 2010 for hurling a rock at the then Japanese ambassador to Seoul.

Kim told investigators the ambassador was the "symbolic" target of his opposition to the annual US-South Korea military exercises, which he blames for blocking dialogue with North Korea.

The joint drills have long been condemned by Pyongyang as rehearsals for invasion.

Kim has visited the North seven times and once tried to erect a memorial in Seoul to the late North Korean leader Kim Jong-Il after his death in 2011.

North Korea has rejected accusations that it may have been behind the attack on Lippert as a "vicious" smear campaign.