The Office of the UN High Commissioner for Human Rights said the deployment of two high-ranking commanders represented a worrying escalation in the situation.

"Alarming reports indicate that there has been substantial deployment of heavy weapons and troops by the Myanmar military, the Tatmadaw, over the past few weeks," spokeswoman Ravina Shamdasani told reporters in Geneva.

The deployments have been to Kanpetlet and Hakha townships in Chin State; Kani and Monywa townships in the central Sagaing region; and Gangaw township in Magway region, she said.

The internet has also been shut down, she added.

Myanmar has been mired in crisis since the army ousted Aung San Suu Kyi's government in February, sparking mass protests and a bloody crackdown.

"We are gravely concerned by these developments, particularly given the intensifying attacks by the military that we have documented over the past month in these areas," said Shamdasani.

The attacks include "killings, raiding of villages and burning of houses, apparently to seek out armed resistance elements and as reprisals against villages perceived to be sympathetic to people's defence forces or ethnic armed groups".

"There have been reports of mass arrests, as well as torture and summary executions. Military 'clearance operations' have involved the use of artillery barrages and airstrikes against villages," said Shamdasani.

"The internet shutdowns have also meant that millions of people have lost the means to communicate and obtain information necessary for their survival, and cast an information blackout over potential abuses."

She called on states with influence over the junta to urge them to "de-escalate the situation, protect civilian lives and property, and ensure access to humanitarian aid for those who need it".