The military has stepped up its use of lethal force to quash mass demonstrations against a February 1 coup which ousted civilian leader Aung San Suu Kyi.

At least 738 people have been killed and 3,300 are languishing in jails as political prisoners, according to a local monitoring group.

"Horrified to learn that... the junta's attacks have already left nearly a quarter (of a) million Myanmar people displaced, according to sources," UN Special Rapporteur on the situation of human rights in Myanmar Tom Andrews tweeted on Wednesday.

"The world must act immediately to address this humanitarian catastrophe."

Free Burma Rangers, a Christian aid group, estimated last week at least 24,000 people were displaced in northern Karen state amid military ground attacks and airstrikes earlier in the month.

Karen National Union brigade five spokesperson Padoh Mann Mann said Wednesday that more than 2,000 Karen people have now crossed Myanmar's border into Thailand and that thousands more are internally displaced.

"They all hide in the jungle nearby their villages," he said.

Amid mounting violence, South East Asian leaders and foreign ministers are set to hold talks on the Myanmar crisis in Jakarta on Saturday.

Coup leader Min Aung Hlaing's expected involvement in the summit has angered activists and human rights groups.

"Min Aung Hlaing, who faces international sanctions for his role in military atrocities and the brutal crackdown on pro-democracy protesters, should not be welcomed at an intergovernmental gathering to address a crisis he created," Human Rights Watch's Brad Adams said.

Overnight, authorities released freelance video journalist Ko Latt, who had been held in custody for a month in the capital Naypyidaw.

At least 70 reporters have been arrested since the coup and 38 are in detention, according to Reporting ASEAN.