The court on Friday sentenced journalist Danny Fenster to 11 years in prison with hard labour, despite calls by the United States and rights groups for his release.

It was the harshest punishment yet among seven journalists known to have been convicted since Myanmar's military ousted the elected government of Aung San Suu Kyi in February's coup.

Fenster, the managing editor of online magazine Frontier Myanmar, still faces additional terrorism and treason charges under which he could receive up to life imprisonment.

The court found him guilty of spreading false or inflammatory information, contacting illegal organisations, and violating visa regulations, lawyer Than Zaw Aung said.

The harsh penalty is the ruling military's latest rebuff of calls from around the world for a peaceful end to Myanmar's political crisis.

The government is refusing to co-operate with an envoy appointed by Southeast Asian governments to mediate a solution, and has not bowed to sanctions imposed by several Western countries.

US State Department spokesman Ned Price called Fenster's sentencing "an unjust conviction of an innocent person".

"The United States condemns this decision," Price said in a statement.

"We are closely monitoring Danny's situation and will continue to work for his immediate release.

"We will do so until Danny returns home safely to his family."

UN human rights chief Michelle Bachelet said Fenster's conviction and harsh sentence "is emblematic of the wider plight of journalists in Myanmar who have been facing constant repression since the February 1 military coup".

According to Bachelet, at least 126 journalists, media officials or publishers have been detained by the military since they seized power and 47 remain in detention, including 20 charged with crimes.

"Journalists have been under attack since February 1, with the military leadership clearly attempting to suppress their attempts to report on the serious human rights violations being perpetrated across Myanmar as well as the extent of opposition to the regime," Bachelet said.

"Myanmar has quickly reverted to an environment of information control, censorship and propaganda seen under military regimes in the past."

UN Secretary-General Antonio Guterres supports Bachelet's views, and reiterated that journalists everywhere, including those in Myanmar, must be allowed to work without harassment and that reporting facts "is not and must not be seen as a crime", UN deputy spokesman Farhan Haq said.

The army's takeover was opposed by widespread peaceful protests that were put down with lethal force.

Security forces killed more than 1200 civilians and arrested about 10,000 others, according to the Assistance Association for Political Prisoners.

Armed resistance has since spread, and UN experts and other observers fear the incipient insurgency could slide into civil war.

Australian Associated Press