A South Korean monk is in critical condition after setting himself ablaze to protest against the country's deal with Japan on compensation for wartime sex slaves.

The 64-year-old Buddhist monk suffered third-degree burns across his body and serious damage to vital organs following his self-immolation during a rally late on Saturday in the capital, Seoul, calling on impeached President Park Geun-hye to stand down, police said.

He is unconscious and unable to breathe on his own, an official from the Seoul National University Hospital told The Associated Press news agency on Sunday on condition of anonymity.

Police said in his notebook, the monk called Park a "traitor" over her government's 2015 agreement with Japan that sought to settle a long-standing row over South Korean women who were forced into sexual slavery by Japan's World War II military.

According to The Korea Times, he left a memo at the scene demanding the nullification of the contentious settlement.

"Please don't make my death worthless," he reportedly wrote.

Rising tensions

Under the agreement, Japan pledged to fund a Seoul-based foundation set up to help support the victims. South Korea, in exchange, vowed to refrain from criticising Japan over the issue and try to resolve the Japanese grievance over a bronze statue representing wartime sex slaves in front of its embassy in Seoul.

Yet, the deal continues to be criticised in South Korea because it was reached without approval from victims, and opponents say it does not go far enough in holding Japan responsible for its wartime abuses.

Students have been holding sit-in protests next to the Seoul statue for more than a year over fears the government might try to remove it.

On Friday, Japan announced it would recall its ambassador to South Korea and suspend economic talks in response to the placing of another "comfort woman" statue representing wartime sex slaves in front of its consulate in the Korean port city of Busan.

Japanese Prime Minister Shinzo Abe urged South Korea to remove the statues and implement the 2015 agreement.

"It has been mutually confirmed that this is a final and irreversible agreement. Japan has sincerely fulfilled its obligation," Abe said on a NHK news talk show aired on Sunday. He said Japan had already paid one billion yen ($8.5m) in compensation.

"Next, I think South Korea must firmly show its sincerity," he said, adding that the agreement should be implemented regardless of leadership change as a "matter of credibility".

At the time of the deal, Seoul said there were 46 surviving South Korean victims.