Myanmar's military junta has sentenced a close aide of ousted civilian leader Aung San Suu Kyi to 20 years in prison for treason, according to the man's lawyer.

Since a February 1 coup, Myanmar has been in turmoil, with nationwide protests and more than 1,100 people killed by security forces, according to a local monitoring group.

"U Win Htein was sentenced to 20 years' imprisonment under section 124a by a special court," lawyer Myint Thwin told AFP on Friday, adding that they would appeal his sentence.

The former member of parliament is the first high-ranking member of Ms Suu Kyi's National League for Democracy to be sentenced by the junta after a trial.

The 79-year-old is a longtime political prisoner, who has spent long stretches of time in and out of detention for campaigning against military rule.

Australian economist Sean Turnell, another close adviser to Ms Suu Kyi, was also arrested in the days following the military coup. 

Last month, Mr Turnell faced court for the first time after being charged under the country's official secrets law. 

Exact details of the charges he faces are unknown, but Myanmar state television has said he had access to "secret state financial information" and had tried to flee the country. 

However, Australian consular officials have been kept out of the courtroom, raising concerns his case will not be monitored and fuelling doubts about whether he will receive a fair trial.

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

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".

Ms Suu Kyi also faces a raft of charges that could see her jailed for decades, from illegally importing walkie talkies to flouting COVID-19 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 Ms 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 the media.

AFP/ABC