"I appeal to this Council to consider all available tools to take collective action and do what is right, what the people of Myanmar deserve and prevent a multi-dimensional catastrophe in the heart of Asia," special envoy Christine Schraner Burgener told the closed-door session, according to remarks obtained by AFP.

She said she remained open for dialogue with the junta but added: "If we wait only for when they are ready to talk, the ground situation will only worsen. A bloodbath is imminent."

More than 520 people have died since the military overthrew elected leader Aung San Suu Kyi on February 1, thereby halting Myanmar's decade-old experiment in democracy, according to figures from monitors confirmed by Burgener to the Security Council.

Amid the turmoil, the military on Saturday launched the first airstrikes in Karen state in 20 years after a rebel group seized a military base -- raising fears of a return to armed ethnic conflict in the ethnically diverse nation.

"The military's cruelty is too severe and many (armed ethnic fighters) are taking clear stances of opposition, increasing the possibility of civil war at an unprecedented scale," Burgener said.

"Failure to prevent further escalation of atrocities will cost the world so much more in the longer term than investing now in prevention, especially by Myanmar's neighbors and the wider region."

Britain called the emergency session of the Security Council after the military dramatically ramped up its use of lethal force against protesters over the weekend.

Linda Thomas Greenfield, the US ambassador to the United Nations, raised the possibility of action if the military does not give up power.

"We're hopeful that the situation will eventually resolve and the military will go back to their barracks and allow the democratically elected government to take its place," she told reporters.

"But if they do not do that, and they continue the attacks that they were making on civilian populations, then we have to look at how we might do more," she said.

The United States has already imposed targeted sanctions and suspended a trade pact with Myanmar.

Ambassador T.S. Tirumurti of India, which has warm relations both with the United States and the Myanmar military, wrote on Twitter that he emphasized at the meeting "our steadfast commitment to democratic transition" and welcomed diplomatic efforts by Southeast Asian nations.