Myanmar has seen weeks of mass protests demanding the military release civilian leader Aung San Suu Kyi, who was detained in a February 1 coup.

Authorities have in response steadily stepped up their use of force, using tear gas, water cannon, rubber bullets and, increasingly, live rounds.

Sunday was the bloodiest day since the February 1 military takeover, with the UN saying that at least 18 protesters were killed across the country. AFP independently confirmed 11 deaths.

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

"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 critical condition," he said, adding that police had initially deployed tear gas and rubber bullets before doubling back with live rounds.

A doctor who treated the patients in a local hospital confirmed the number of people in critical condition.

"One was hit in his thigh and he's now under operation. Another one got hit in the abdomen and he requires blood transfusions... Another one got hit in the chest," he told AFP.

- 'No mercy, just bullies' -

The bloodshed came the same day as the funeral was held in the commercial capital of Yangon for a 23-year-old student who died Sunday.

The mourners sang a revolutionary song as the coffin carrying Nyi Nyi Aung Htet Naing moved through a sea of thousands to an altar. Some climbed trees to catch a glimpse of the procession.

"No mercy, just bullies -- dead bodies are here and there," the mourners sang in unison as they flashed a three-finger salute. "Oh, the brave heroes who died for democracy."

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

In San Chaung township -- which saw crackdowns in recent days -- hundreds of police came out in force.

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

About 15 minutes away near popular shopping area Yuzana Plaza, local media live-streamed protesters chanting as what appeared to be smoke from tear gas whizzed their way.

"We threw wet blankets to the protesters for them to cover the canisters to prevent the tear gas (from spreading)," said one resident living by the plaza, who added that she saw some protesters detained by authorities.

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 some 900 are still behind bars.

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

A Myanmar journalist was arrested overnight in the southern city of Myeik -- the latest among the country's press corps who have seen themselves targeted by security forces.

- 'Appalled' -

The unrest continued as foreign ministers from the 10-member Association of Southeast Asian Nations bloc were due to meet virtually, including a Myanmar representative.

Ahead of the meeting, some regional powers broke with diplomatic traditions and issued unusually harsh rebukes to Myanmar's junta.

"To use lethal force against civilians and unarmed demonstrators, I think it is just not acceptable," said Singapore's Prime Minister Lee Hsien Loong in an interview Tuesday with the BBC, calling Myanmar's return to military rule "an enormous tragic step".

"There is no future that way."

As Myanmar's biggest foreign investor, the prosperous city-state is seen as having some leverage over the generals.

While no joint statement has been released yet, Malaysian Foreign Minister Hishammuddin Hussein has called for "the prompt and unconditional release of detained political leaders in Myanmar", according to a speech released by its ministry.

Observers are sceptical about what difference the bloc can make, pointing to its policies of non-interference and making decisions based on consensus -- shortcomings acknowledged by Singapore's Prime Minister in his BBC interview.

"Outsiders have very little influence on this," he said.