Myanmar has been in turmoil since the February coup ousted civilian leader Aung San Suu Kyi. Security forces have launched a bloody crackdown on dissent that has killed more than 1,200 people, according to a local monitoring group.

Junta chief Min Aung Hlaing received former UN ambassador Richardson in the capital Naypyidaw, where they "exchanged views and discussed... COVID-19 vaccine assistance from the US to Myanmar", a military-run news website said.

Richardson is in Myanmar on a "private humanitarian mission", his organisation the Richardson Center said in a statement announcing his departure.

The military website made no mention of American journalist Danny Fenster, who was detained in May and has since been charged with encouraging dissent against the military and unlawful association.

He faces six years in jail if convicted on both charges.

Richardson, a former governor of New Mexico, has negotiated "the release of hostages and American servicemen in North Korea, Cuba, Iraq and the Sudan", according to his centre's website.

His trip comes with the junta increasingly isolated and smarting from a rare snub by regional bloc ASEAN, which barred Min Aung Hlaing from a summit last month over his failure to engage with dissidents.

Richardson's last visit to Myanmar was in 2018, as part of a panel created to advise on violence in Rakhine state -- the site of a 2017 army crackdown that triggered an exodus of more than 700,000 stateless Muslim Rohingya.

But he abruptly resigned after the trip, accusing Suu Kyi of lacking "moral leadership" over the crisis.

Richardson also said he stepped down due to fears the committee would "whitewash" the causes of the Rohingya crisis, over which Myanmar is now being investigated on genocide charges.

Min Aung Hlaing, now the junta chief, was head of the armed forces during the 2017 crackdown.