Myanmar's ousted leader Aung San Suu Kyi has been convicted again by a military-controlled court and faces an even longer time in prison.

A source familiar with proceedings of the closed court said Aung San Suu Kyi and her Australian economic advisor Sean Turnell were sentenced to three years. Both had pleaded not guilty to allegations that they violated Myanmar's official secrets act.

The conviction was Aung San Suu Kyi's 12th since she was detained following a military coup early last year. It was the second this month, after she was also found guilty of election fraud. She now faces 23 years in prison.

Turnell and three members of her cabinet also received three-year sentences.

Australia's Foreign Ministry has lodged a protest. It said in a statement the government rejects the ruling against Turnell and "calls for his immediate release."

Australia repeatedly demanded Turnell's release during his almost 20 months of detention.