A top Karen National Union leader appeared in court yesterday on charges of unlawful association and treason, only three weeks after the Burmese government agreed to a ceasefire with its long-time foe.

Mahn Nyein Maung was brought to a courtroom inside the compounds of Rangoon’s notorious Insein prison where hundreds of political prisoners have been tried over the years. If found guilty of treason, he faces a possible death sentence or life imprisonment.

His lawyer, Kyi Myint, said that judges yesterday heard two prosecution statements, including one from a police investigator. Both charges stem from his role in the KNU, which has been battling the Burmese government for more than six decades – originally the group was fighting for an independent state, although it now calls for a federalised Burma.

A tentative ceasefire agreement on 12 January appears not to have helped Mahn Nyein Maung’s case – the treason charges were brought only recently, after he had already been in detention for seven months on immigration charges. It was only during his time in Insein prison that police discovered he was a KNU member.

His lawyer said the main evidence used by the prosecution was documents they discovered on the internet, something he claims does not qualify as concrete proof of his guilt.

“We questioned the court over whether they would accept in a trial if we submitted material from the internet as evidence against [the government],” Kyi Myint said. “At the end of the hearing, the court decided to throw out the [internet material] as evidence – that is exceptional.”

Mahn Nyein Maung has already spent time in prison on the Coco Islands, 300 kilometres south of Rangoon in the Andaman Sea. After nearly a decade in jail on the islands, he and another prisoner managed to escape on rafts, but were apprehended on the Burmese mainland and thrown back in jail.

The government has been warned that the trial could derail fragile peace talks with the KNU, which appear to be on tenterhooks in the wake of renewed attacks by Burmese troops in Karen state last week.

Although death sentences are still awarded by courts in Burma, no one has been formally executed for a number of years. Two whistleblowers sentenced to death in January 2010 for leaking details of secretive senior-level governmental visits to North Korea and Russia remain in prison.