Danny Fenster, managing editor of Frontier Myanmar, was detained at Yangon International Airport as he attempted to leave the country.

He is currently on trial for allegedly encouraging dissent against the military, which carries a maximum three-year jail sentence.

During the latest hearing at Insein prison in Yangon on Monday, he was hit with another charge of unlawful association, his lawyer Than Zaw Aung said.

Conviction under the colonial-era law also carries a maximum sentence of three years in jail.

It has previously been used to target journalists contacting Myanmar's myriad ethnic armed groups fighting the state for increased autonomy and control over natural resources.

Fenster's second trial is expected to start on October 15, Than Zaw Aung said.

His client was "in good health, but he lost weight a little bit", he added.

In Washington, a State Department spokesperson said the United States was "deeply concerned" and added, "Journalism is not a crime."

"We are closely monitoring Danny's situation and continue to press Burma's military regime to release Danny immediately," the spokesperson said, using Myanmar's former name.

"We will do so until he returns home safely to his family," he said.

"Danny's detention, and that of so many others, is a sad reminder of the continuing human rights and humanitarian crisis facing the country."

Fenster, 37, had been working for Frontier for around a year and was heading home to see his family when he was detained on May 24.

He is believed to have contracted Covid-19 during his detention, family members said during a conference call with American journalists in August.

Myanmar has been in turmoil since the military seized power in a February 1 coup and ousted Aung San Suu Kyi's elected government.

The press has been squeezed as the junta tries to tighten control over the flow of information, throttling internet access and revoking the licences of local media outlets.

More than 100 journalists have been arrested since the putsch, according to Reporting ASEAN, a monitoring group.

It says 48 are still in detention.