The number of pro-democracy protesters killed in the ongoing demonstrations in Myanmar against the February 1 military coup has surpassed the grim 500 mark, a monitoring group said on Tuesday.

The Assistance Association for Political Prisoners (AAPP) puts the nationwide death toll at 510, after another 14 people lost their lives at the hands of security forces on Monday, reports dpa news agency.

The deteriorating situation in Myanmar is alarming the international community, especially after 110 deaths were reported on March 27 alone, with the European Union calling it "a day of terror".

The latest crackdown was in Yangon's South Dagon township, where witnesses say that, during the past two days, the military has carried out a campaign of destruction that laid waste to whole neighbourhoods.

The General Strike Committee of Nationalities, one of the main groups behind the protests, on Monday urged Myanmar's ethnic armed groups to stand on the side of protesters.

On Tuesday, three such groups heeded the call.

In a joint statement, they "strongly denounced" the military's actions and said they share their sympathies with the family members of those who have lost their lives fighting for Myanmar.

The military must immediately stop their attacks and engage in political dialogue, the Myanmar National Democratic Alliance Army, Palaung State Liberation Front and the Arakan Army said in the statement.

"If Myanmar's military doesn't do the above mentioned facts and continue (the killings), our three partner groups will cooperate with all nationalities who are joining Myanmar's spring revolution in terms of self-defence," it added.

The military seized control of the South-East Asian country after an election which former de facto leader Aung San Suu Kyi's ruling National League for Democracy party won by a landslide.

The Army has had Suu Kyi under house arrest since February 1.