Special envoy Noeleen Heyzer "is deeply concerned by the continued escalation of violence in Kayin State and other parts of Myanmar," she said in her first statement since she was appointed in October.

Nationwide protests against the military's February coup have been met with a bloody crackdown, with more than 1,300 people killed and over 11,000 arrested, according to a local monitoring group.

Diplomatic efforts to resolve the crisis led by the UN and the Association of Southeast Asian nations have so far made little headway, with the generals refusing to engage with opponents of their power grab.

In October UN Secretary-General Antonio Guterres named Singaporean sociologist Heyzer as the new special envoy to Myanmar, replacing Swiss diplomat Christine Schraner Burgener.

On Sunday a UN official said he was "horrified" by credible reports that at least 35 civilians were killed and their bodies burned in an attack on Christmas Eve in the east of the country, and demanded the government launch an investigation.

Two workers for non-profit group Save the Children remain missing after their vehicle was among several that were attacked and burned in the incident in eastern Kayah state.

The charity said Monday it was still investigating the incident.