Danny Fenster, 37, who works as managing editor for Frontier Myanmar, was said to have been detained by local authorities at Yangon International Airport on Monday morning shortly before boarding a flight to Kuala Lumpur.
His brother, Bryan Fenster, said on Facebook the family is “absolutely stunned and extremely confused as to why Dan was detained”.
According to Frontier Myanmar, Mr Fenster was transferred to the Insein Prison in Yangton. He is the third foreign journalist to be detained since a military coup in February, according to a count kept by Reuters.
“We do not know why Danny was detained and have not been able to contact him since this morning. We are concerned for his wellbeing and call for his immediate release,” Frontier Myanmar said in a statement.
“Our priorities right now are to make sure he is safe and to provide him with whatever assistance he needs.”
An official with the United States Department of State told CNN that it is “aware of reports of a US citizen detained in Burma”, using the country’s traditional nomenclature of its Burman ethic group before it was renamed in 1989 by the ruling military junta.
“We take seriously our responsibility to assist US citizens abroad, and are monitoring the situation. Due to privacy considerations, we have no further comment," the official told CNN.
In February, the military took control of the country in a coup that wrested power from the semi-democratic government.
A Japanese journalist who was detained on 19 April while covering the aftermath of the coup was released on 14 May as part of a diplomatic deal, according to AFP.
Yuki Kitazumi, who was detained at the same prison as Mr Fenster is reportedly being held, said on his return to Tokyo that he collected “harrowing testimonies” from fellow inmates.
“Some are deprived of meals for two days, others are questioned whilst being threatened with a weapon, or beaten if they try to deny [the allegations],” he said, according to AFP.
“Thanks to my Japanese nationality, I was able to escape this type of treatment, but the reality is that many Burmese are being tortured.”