The news organization also said it understands Fenster has been transferred to Insein Prison near Yangon. Insein is one of the country's most notorious prisons, known for its deplorable conditions.

Speaking to CNN Monday evening, Bryan Fenster said the family doesn't have much information on his brother's situation.

"I can only assume being a journalist in a country that's run by the military who wants to control the narrative, he was flagged being a journalist when he was at the airport. Can't begin to imagine why it happened," he said. "He was on valid work papers, valid visas, passports, everything. He was voluntarily leaving the country to come visit family, so we can't see what the issue is."

Bryan Fenster said the family are working with elected officials in Michigan and said the US state department is aware of his brother's case.

"We're very hopeful with all the support, we'll be getting him out as soon as possible," he said.

A US Department of State official, who used another name for Myanmar, said the agency was monitoring the situation. "We are aware of reports of a US citizen detained in Burma. We take seriously our responsibility to assist US citizens abroad, and are monitoring the situation. Due to privacy considerations, we have no further comment."

Fenster is among a number of foreign journalists to be detained in Myanmar since the coup.

Japanese journalist Yuki Kitazumi was arrested in April and charged with spreading false information. He was held in Insein Prison until the junta released him two weeks ago and returned him to Tokyo.

Many detained reporters are charged with crimes under section 505a -- a law amended by the military that makes it a crime punishable by up to three years in prison for publishing or circulating comments that "cause fear" or spread "false news."

In a statement, the Foreign Correspondents Club of Thailand (FCCT) said it was not clear whether Fenster had been charged with any offense.

"The arrests of journalists and the violence used by the security forces on anyone caught trying to report or record their actions, constitute an extraordinary attack on freedom of expression in Myanmar, and should be widely condemned," the FCCT statement said.

In its nightly news bulletin, Myanmar state TV aired the first images of Suu Kyi since the coup. She was detained in the early hours before the military seized power and had not been seen in public since.

The images showed Suu Kyi in the dock with her hands in her lap and wearing a surgical mask. Beside her sat ousted President Win Myint and the former mayor of the capital Naypyidaw, Myo Aung.

Suu Kyi faces a range of charges, from illegally possessing walkie-talkie radios to violating the official secrets act -- the most serious charge which carries a potential prison sentence of up to 14 years.

Last week, Myanmar's junta-appointed election commission said it would dissolve the ousted National League for Democracy (NLD) party because of what it claims was election fraud. The military overthrew the NLD government alleging mass voter discrepancies, though the electoral commission at the time rejected the army's complaints.

According to her lawyer Khin Maung Zaw, Suu Kyi struck a defiant note, saying of the NLD: "Our party grew out of the people so it will exist as long as people support it."