NAYPYIDAW, Sept 29 (Reuters) - A court in military-ruled Myanmar on Thursday sentenced deposed leader Aung San Suu Kyi and her former economic adviser, Australian Sean Turnell, to three years in prison, a source familiar with the proceedings said.

Both had been charged with violating an official secrets act, which carries a maximum sentence of 14 years, and had pleaded not guilty.

"Three years each, no hard labour," said the source, who declined to be identified due to the sensitivity of the issue.

Suu Kyi, Turnell, and several members of her economic team are among thousands arrested since the junta overthrew an elected government in a coup early last year, including politicians, lawmakers, bureaucrats, students and journalists.

Nobel laureate Suu Kyi has already been sentenced to more than 17 years in prison in separate cases, mostly related to corruption charges. She denies all allegations against her.

Turnell, who is also a professor of economics at Macquarie University in Australia, has also been in detention since a few days after the coup.

The Australian Prime Minister's Office and Foreign Ministry did not immediately respond to a request for comment. Foreign Minister Penny Wong has previously said Canberra rejected a court decision to put Turnell on trial.

Thursday's sentencing took place in a closed court in the capital, Naypyitaw. The defendants' exact offence under the official secrets act remains unclear, though a source previously said Turnell's offence "relates to an allegation that he had government documents".

A junta spokesperson did not answer calls seeking comment on Thursday. The junta insists Myanmar's courts are independent and those arrested are receiving due process.