Security forces opened fire on unarmed demonstrators in four cities on Sunday, with the UN saying it had credible information at least 18 people had died.

One person among a group of protesters crouching behind rubbish bins and other makeshift shields in Yangon, the commercial capital, was shot and had to be dragged away by others, according to video footage filmed by AFP.

"We strongly condemn the escalating violence against protests in Myanmar and call on the military to immediately halt the use of force against peaceful protesters," Ravina Shamdasani, spokeswoman for the UN human rights office, said.

AFP independently confirmed eight deaths in Sunday's violence, although there were fears the toll could be much higher.

The Assistance Association for Political Prisoners, a reliable monitoring group, estimated that about 30 people have been killed by security forces since the coup on February 1.

Suu Kyi, 75, was detained before dawn on that day, and has not been since in public since.

The military has justified its takeover, ending a decade-long democratic experiment, by making unfounded allegations of widespread fraud in last November's national elections.

Suu Kyi's National League for Democracy won the election in a landslide.

The generals have hit Suu Kyi with two charges the international community widely regards as frivolous -- relating to importing walkie talkies and staging a campaign rally during the pandemic.

- Secret detention -

Western powers have repeatedly condemned the generals and imposed sanctions, but the military has responded to the growing pressure at home and abroad by escalating its use of force.

Suu Kyi is reported to be being held in, Nyapyidaw, the isolated capital city that the military built during a previous dictatorship.

Suu Kyi's lawyer, Khin Maung Zaw, told AFP he had not been able to speak to her ahead of Monday's scheduled court hearing, which is for preliminary matters and where she may appear only via video link.

He said he hoped the court would formally approve his status as her defence lawyer.

"It will be very unfair for her... not to grant a lawyer immediately," he told AFP.

The veteran human rights lawyer said the hearing would focus on case management and the timeline for the trial.

Hundreds of thousands of people have taken to streets regularly over the past month to oppose the coup.

While the military has steadily increased the type of force used to try to contain the uprising, beginning with tear gas and water cannons, the weekend's violence was the biggest escalation.

- Deadly crackdown -

Unarmed protesters faced live rounds, rubber bullets and tear gas.

Three men were killed and at least 20 others injured when security forces moved on a rally in the southern coastal hub of Dawei.

Local rescue worker Pyae Zaw Hein said the trio were "shot dead with live rounds", while the injured were hit by rubber bullets.

But on Monday morning, protesters again took to the streets in Dawei.

Hundreds of people were arrested over the weekend with many in Yangon taken to Insein Prison, where many of Myanmar's leading democracy campaigners have served long jail terms under previous dictatorships.

At least one journalist documenting Sunday's assaults by security forces was beaten and detained further north in Myitkyina, a city at the headwaters of the Irrawaddy river, according to local outlet The 74 Media.

Another reporter was shot with rubber bullets while covering a protest in the central city of Pyay, their employer said.

Several journalists documenting Saturday's assaults by security forces were detained, including an Associated Press photographer in Yangon.

The United States has been one of the most outspoken critics of the junta, and Secretary of State Antony Blinken again reacted with horror after Sunday's violence.

"We condemn the Burmese security forces' abhorrent violence against the people of Burma & will continue to promote accountability for those responsible," Blinken tweeted, using the country's old name.