(AFP) - Britain has called for the UN Security Council to meet Friday on Myanmar, diplomatic sources told AFP, as security forces have steadily stepped up their use of violence against anti-coup demonstrators in the Southeast Asian nation.

Authorities fired live rounds and tear gas at protesters again on Tuesday, leaving at least three people critically injured as international pressure has grown since the military's February 1 ouster of civilian leader Aung San Suu Kyi.

The UN meeting would be behind closed doors at 1500 GMT under London's proposal, the sources said, as were the Council's discussions a day after the coup.

Soldiers and police have steadily stepped up their use of force, deploying tear gas, water cannons, rubber bullets and, increasingly, live rounds after weeks of mass protests.

Sunday was the bloodiest day since the military takeover, with the United Nations saying at least 18 protesters were killed across the country. AFP independently confirmed 11 deaths.

Another rally turned violent Tuesday in the northwestern town of Kale, where security forces opened fire on protesters, according to medics who witnessed events and treated those wounded.

- 'Tear gas and shooting' -

"About 20 people were injured in a morning crackdown by police and soldiers in Kale," said a rescue worker, speaking on condition of anonymity for fear of repercussions.

"Three... were hit by live rounds and are in a critical condition," he said.

Police had initially deployed tear gas and rubber bullets before doubling back with live rounds, he added.

As the violence escalates, Myanmar's military junta and the envoy sent by its toppled civilian government have launched contradictory claims over who represents the country at the United Nations.

Ambassador Kyaw Moe Tun spectacularly broke with the military before the General Assembly on Friday in an emotional plea for help to restore ousted civilian leadership, which prompted the junta to announce his removal.

Both he and the junta now claim to represent the country at the body, with the UN saying it is evaluating what action to take on the competing claims..

"We will continue to oppose the military coup and we will continue to support the restoration of Burma's democratically elected civilian government," the State Department spokesman said.

The bloodshed on the ground in Myanmar came on the same day as a funeral in the commercial capital of Yangon for a student who died Sunday.

Mourners sang a revolutionary song as the coffin carrying Nyi Nyi Aung Htet Naing moved through a sea of thousands to an altar.

"No mercy, just bullies -- dead bodies are here and there," the mourners sang in unison as they flashed a three-finger salute that has become a symbol of opposition to the junta.

Protests also continued in several neighbourhoods of Yangon on Tuesday, with demonstrators wearing hard hats and wielding improvised home-made shields.

In San Chaung township hundreds of police came out in force.

"They used tear gas and were shooting as well," said one resident.

More than 1,200 people have been arrested, charged and sentenced since the coup, according to the Assistance Association for Political Prisoners monitoring group, of which about 900 are still behind bars.

But the real number is likely far higher -- state-run media reported that on Sunday alone more than 1,300 people were arrested.

State-broadcaster MRTV said late Tuesday that 511 people in detention had been released in Yangon.

Reporters Without Borders says at least 10 journalists are in jail and 26 have been arrested since the coup.

- 'Not acceptable' -

The unrest came as foreign ministers from the 10-member Association of Southeast Asian Nations bloc held a virtual meeting which included a Myanmar representative.

Some regional powers broke with diplomatic convention and issued unusually harsh rebukes of Myanmar's junta.

"To use lethal force against civilians and unarmed demonstrators, I think it is just not acceptable," Singapore Prime Minister Lee Hsien Loong told the BBC.

After the meeting, Indonesia's foreign minister Retno Marsudi expressed concern over rising violence and deaths as well as frustration over a lack of cooperation from the Myanmar regime.

"It takes two to tango," she said, calling for the restoration of democracy and the release of political detainees.

Philippines foreign secretary Teodoro Locsin said the first step should be the immediate release of Suu Kyi.

"The hurt of the small finger is felt by the whole body. Myanmar is not a small finger but a big part of the two hands that together make up the family of ASEAN 10," he said.

Observers are sceptical about what difference the bloc can make, pointing to its policies of non-interference and making decisions based on consensus.

© Agence France-Presse