Kyaw Moe Tun was fired by the junta on Saturday, a day after urging countries at the 193-member UN General Assembly to use "any means necessary" to reverse the February 1 coup that ousted the nation's elected leader Aung San Suu Kyi.

In Washington, Myanmar's embassy also signaled a break with the junta on Thursday, issuing a statement decrying the deaths of civilian protesters and calling on authorities to "fully exercise utmost restraint".

Police in Myanmar broke up demonstrations in several places with tear gas and gunfire on Thursday as protesters took to the streets again, undeterred by the rising death toll in a crackdown on coup opponents.

"Instead of demonstrating respect for the rule of law, pursuing dialogue and refraining from violence, the military has dramatically accelerated violence against the people of Burma," US Ambassador to the United Nations Linda Thomas-Greenfield said.

"This is simply unacceptable."

On Sunday, the Myanmar UN mission said Kyaw Moe Tun's deputy, Tin Maung Naing, would become the acting UN envoy.

On Monday, Kyaw Moe Tun formally staked his claim to remain the country's legitimate representative - a job he has held since October - in a letter to the United Nations.

The rival claims raised the prospect of the UN General Assembly having to address the issue.

On Wednesday, the Myanmar UN mission told the United Nations Tin Maung Naing had resigned and Kyaw Moe Tun remained the country's ambassador.

It said the note it sent on Sunday "shall be ignored".

Myanmar's representation at the United Nations could become an issue again if the junta tries to appoint a new ambassador.

The UN special envoy on Myanmar, Christine Schraner Burgener, has warned that no country should recognize or legitimize the Myanmar junta.

The UN Security Council will discuss Myanmar on Friday.

The 15-member council has voiced concern but stopped short of condemning the coup due to opposition from Russia and China.

Australian Associated Press