The death toll in the Myanmar military's crackdown on protesters has passed 500, as armed rebel groups on Tuesday threatened the junta with retaliation if the bloodshed does not stop.

World powers have ramped up their condemnation of the military's campaign against the anti-coup movement that is demanding the restoration of the elected government and the release of civilian leader Aung San Suu Kyi.

Washington suspended a trade pact with Myanmar and UN chief Antonio Guterres called for a united global front to pressure the junta after more than 100 protesters were killed in a bloody weekend.

Adding to that pressure campaign, a trio of ethnic rebel groups on Tuesday condemned the crackdown and threatened to fight alongside protesters unless the military reined in its violence.

Daily rallies across Myanmar by unarmed demonstrators have been met with tear gas, rubber bullets and live rounds.

The Assistance Association for Political Prisoners (AAPP) said it had confirmed a total of 510 civilian deaths but warned the true toll was probably significantly higher.

On Tuesday, protesters in Yangon emptied rubbish bags in the streets as part of the latest action.

— Rebel warning —

Three of the country's myriad armed ethnic rebel groups — the Ta'ang National Liberation Army, the Myanmar Nationalities Democratic Alliance Army and the Arakan Army (AA) — issued a joint statement threatening retaliation.

"If they do not stop, and continue to kill the people, we will cooperate with the protesters and fight back," the statement said.

If rebel groups take up arms, Debbie Stothard at the International Federation for Human Rights (FIDH) warned that the situation could degenerate towards civil war.

Two dozen ethnic minority rebellions have flared in Myanmar since independence from British colonial rule in 1948, fighting over autonomy, ethnic identity, drugs and natural resources.

The military has sought to cut deals with some armed groups and earlier this month took the AA off the list of terrorist organisations.

But over the weekend it launched airstrikes in eastern Karen state — the first such strikes in 20 years — targeting the Fifth Brigade of the Karen National Union (KNU) after the group seized a military base.

An estimated 3,000 people fled through the jungle to seek safety across the border in Thailand, according to local groups.

There were reports that some 2,000 people had been pushed back, though the Thai foreign ministry denied this.

— Global concern —

UN Secretary-General Guterres urged the Myanmar authorities to undertake a "serious democratic transition".

"It is absolutely unacceptable to see violence against people at such high levels, so many people killed," Guterres told a news conference.

"We need more unity… (and) more commitment from the international community to put pressure in order to make sure that the situation is reversed," he said.

US President Joe Biden's administration announced Monday that the 2013 Trade and Investment Framework Agreement, which laid out ways to boost business but was not a fully fledged deal, would remain suspended until democracy is restored.

"The United States strongly condemns the Burmese security forces' brutal violence against civilians," US Trade Representative Katherine Tai said, using Myanmar's former name of Burma.

Saturday saw the military mark its annual Armed Forces Day with a major parade of troops and armour in the capital Naypyidaw.

But the day also saw repression of protests around the country, with at least 107 people killed, including seven children.

Despite the bloodshed, protesters turned out again on Monday, with mourners at funerals defiantly showing the three-fingered salute that has become emblematic of the movement.

The UN Security Council will meet on Wednesday to discuss the situation, diplomatic sources said, after Britain called for emergency talks.

France condemned the violence as "blind and deadly" and China added its voice to a chorus of international concern on Monday, calling for restraint from all sides.

The Kremlin said it was "very concerned" by growing civilian casualties, despite acknowledging it was building ties with the military authorities.

The US, Britain and the EU have all imposed sanctions in response to the coup and crackdown, but so far diplomatic pressure has not persuaded the generals to ease off.

Myanmar bloodshed 'absolutely outrageous', Biden says after deadliest day
Yangon (AFP) March 29, 2021 –

US President Joe Biden has led global condemnation of an "absolutely outrageous" crackdown by Myanmar's junta that left more than 100 people — including several children — dead in the bloodiest day since the coup two months ago.

Soldiers and police have killed hundreds in brutal suppression against weeks of mass protests demanding a restoration of democracy and the release of detained civilian leader Aung San Suu Kyi.

At least 107 more people were killed on Saturday, the United Nations said, as the regime staged a major show of might for Armed Forces Day — an annual parade showcasing Myanmar's military prowess.

"It's absolutely outrageous and based on the reporting I've gotten, an awful lot of people have been killed totally unnecessarily," Biden told reporters on Sunday.

European Union foreign policy chief Josep Borrell said the junta's celebration of its armed forces had been blighted by "a day of horror and of shame".

The latest chorus of international condemnation came after the defence chiefs of the United States, Britain, Japan and nine other countries denounced the Myanmar military.

"A professional military follows international standards for conduct and is responsible for protecting — not harming — the people it serves," the rare joint statement said.

The death toll from crackdowns since the February 1 coup has climbed to at least 459, according to the Assistance Association for Political Prisoners (AAPP) monitoring group.

AAPP said a further 13 people were killed on Sunday, as funerals were held for some of the victims, after the deadliest day of violence in the eight weeks since the coup.

In Mandalay, the country's cultural capital, the family of Aye Ko mourned at a funeral service after he was killed overnight.

"I am very saddened to lose my husband — together with my children I'm heartbroken," his wife Ma Khaing told AFP as she grieved with her four children.

On Monday the British foreign ministry advised its nationals in Myanmar to leave as soon as possible, following "a significant increase in the level of recent violence".

– 'Shameful, cowardly, brutal' –

Despite the dangers, protesters hit the streets again Sunday in parts of commercial hub Yangon and several other cities around Myanmar.

A 16-year-old boy lost his hand in one Yangon neighbourhood while trying to lob back a grenade that security forces had thrown at protesters, a rescue worker said.

The UN put Saturday's death toll at 107 people — including seven children — but expected it to rise further.

"The shameful, cowardly, brutal actions of the military and police — who have been filmed shooting at protesters as they flee, and who have not even spared young children — must be halted immediately," United Nations envoys Alice Wairimu Nderitu and Michelle Bachelet said in a joint statement.

Military-run broadcaster Myawaddy TV gave Saturday's death toll as 45 and said the crackdown was necessary because protesters had used guns and bombs against security forces.

– 'Harmful to state tranquillity' –

An insurgent group in eastern Myanmar's Karen state said they had been targeted in air strikes late Saturday, hours after the ethnic armed group seized a military base.

Hsa Moo, an ethnic Karen and human rights activist, said three people were killed and at least eight wounded.

It was the first air assault in 20 years in the state, and targeted the Karen National Union (KNU) — one of the country's largest non-state armed groups.

Further air strikes on Sunday sent 3,000 people fleeing through the jungle to seek safety across the border in Thailand, the Karen Women's Organization civil society group said.

Saturday's grand parade of troops and military vehicles in the capital Naypyidaw saw junta leader General Min Aung Hlaing defend the coup and pledge to yield power after new elections.

But he also issued a threat to the anti-coup movement, warning that acts of "terrorism which can be harmful to state tranquillity and security" were unacceptable.

Later that evening he entertained dignitaries including Russian deputy defence minister Alexander Fomin at a lavish outdoor dinner.