Myanmar's military sentenced Win Htein, a close aide of ousted civilian leader Aung San Suu Kyi, to 20 years in prison on Friday, his lawyer said, the first high-ranking member of her party to be jailed by a junta court.

Former member of parliament and NLD speaker Win Htein was handed 20 years in jail for treason by a special court in the capital Naypyidaw, lawyer Myint Thwin told AFP.

He added that he would appeal the sentence, and that his client was in good health.

Win Htein, 80, is the first high-ranking NLD member to be sentenced by the junta after a trial.

The former soldier is a longtime political prisoner who has spent long stretches of time in detention for campaigning against military rule.

Considered Suu Kyi's right-hand man, he was long been sought out by international and domestic media for insights into what Myanmar's former de facto leader is thinking.

The wheelchair-bound politician requires oxygen to help him breathe, according to local media, and suffers from hypertension, diabetes, and heart and thyroid diseases.

Ahead of his arrest three days after the coup, he told local media the military putsch was "not wise", and that its leaders "have taken (the country) in the wrong direction".

- Vendetta -

The sentencing was "an indication that this regime does want to throw the book at these NLD leaders", including Suu Kyi, said Richard Horsey of the International Crisis Group.

"They do want to portray them as having been traitorous, corrupt and so on. It's definitely not a good sign for others," he said, although he added it was possible the junta held a vendetta against former soldier Win Htein.

Suu Kyi faces a raft of charges that could see her jailed for decades, from illegally importing walkie-talkies to flouting coronavirus rules.

She testified for the first time in a junta court on Tuesday, four months after being put on trial by the military, a source with knowledge of the case told AFP.

Media have been barred from attending Suu Kyi's trial at the special court in the military-built capital Naypyidaw and the junta recently banned her legal team from speaking to reporters.

Before the ban, her lawyers said the Nobel laureate, 76, would not call any defence witnesses at her incitement trial and she will represent herself.

The junta has threatened to dissolve the NLD and continues to wage a bloody campaign against opponents to its rule.