A court in military-ruled Myanmar on Friday jailed American journalist Danny Fenster for 11 years, his lawyer and his employer said, dealing a blow to US efforts to secure his release.

Fenster, 37, managing editor of online magazine Frontier Myanmar, was found guilty of incitement and violations of immigration and unlawful associations laws, his magazine said, describing the sentences imposed as "the harshest possible under the law".

He is the first Western journalist to be jailed in recent years in Myanmar, where a February 1 coup by the military against Aung San Suu Kyi's elected government ended a decade of tentative steps towards democracy and triggered nationwide backlash.

"There is absolutely no basis to convict Danny of these charges," said Thomas Kean, editor-in-chief of Frontier Myanmar, one of the country's top independent news outlets.

"Everyone at Frontier is disappointed and frustrated at this decision. We just want to see Danny released as soon as possible so he can go home to his family."

Fenster was arrested while trying to leave the country in May and has since been held in Yangon's notorious Insein prison.

His family has repeatedly called for his release.

He was charged with additional and more serious offences of sedition and violations of the terrorism act earlier this week, without an explanation by authorities. Those charges are punishable by a maximum 20 years in prison each.

Fenster's trial had not been made public and a spokesman for the junta did not immediately respond to a request for comment.

'Plain to see'

The American is among dozens of journalists who were detained in Myanmar after protests and strikes erupted following the coup, hampering the military's efforts to consolidate power. Independent media has been accused by the junta of incitement.

The United States has been pushing for Fenster's release. The US embassy in Myanmar did not immediately respond to a request for comment on Friday's verdict.

The State Department had earlier said his detention was "profoundly unjust" and "plain for the world to see", urging the junta to release him immediately.

Myanmar authorities overlooked Fenster in a recent amnesty for hundreds of people detained over anti-junta protests, which included some journalists.

Phil Robertson of the New York-based Human Rights Watch said on Twitter Fenster's jailing was "outrageous and unacceptable" and the charges were trumped up and bogus.

During nearly half a century of harsh military rule, news reporting was tightly controlled by the state but Myanmar's media blossomed after the generals introduced tentative reforms from 2011.

Since the February coup, however, the military has rescinded media licenses, imposed curbs on the internet and satellite broadcasts and arrested dozens of journalists, in what human rights groups have called an assault on the truth.

More than 1,200 civilians have been killed in protests and thousands detained since the coup, according to activists cited by the United Nations.

"A lot of things are going so wrong in this country," Sonny Swe, a journalist and publisher who was jailed under the previous era of military rule, said on Twitter.