"I urge the military authorities to immediately release all journalists being detained in relation to their work," Michelle Bachelet said in a statement.

Her comment came after a Myanmar junta court on Friday sentenced American journalist Danny Fenster to 11 years behind bars on charges of unlawful association, incitement against the military and breaching visa rules, according to his lawyer.

Fenster, who worked for local outlet Frontier Myanmar, was arrested in May at Yangon airport. He is still awaiting a second trial on charges of sedition and terrorism, which could see him jailed for life.

The statement from the UN High Commissioner for Human Rights decried the "harsh sentence", adding that the "closed door, unfair trial is emblematic of the wider plight of journalists in Myanmar".

The military has squeezed the press since taking power in a February coup, arresting dozens of journalists critical of its crackdown on dissent, which has killed over 1,200 people according to a local monitoring group.

With these attacks, Bachelet said the military was "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."

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

The former Chilean president warned that "attacks on journalists and the media further exacerbate the vulnerability of huge sections of society that rely on accurate and independent information."

"With the crackdowns on journalists, Internet shutdowns, restrictions on free access to online and other data sources ... people are being deprived life-saving information," she said.