Yangon, Myanmar - At least 38 people were killed by Myanmar's security forces as they opened fire on peaceful protesters, the United Nations reported on Wednesday, a month after a military coup toppled the government led by Aung San Suu Kyi.

"Today was the blackest day since the coup happened on the first of February," UN special envoy Christine Schraner Burgener said.

"Today, only today, 38 people died," she added.

She described videos showing violence against journalists and an obvious shooting of a protester, calling the footage "very disturbing."

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"It seems that the police used weapons like nine millimetre submachine guns. So live ammunition," Schraner Burgener said.

The UN special envoy called on member states to employ all tools available "to stop this situation" and called for unity among the international community.

"It's up to the member states to take the right measures," she said.

Earlier, there had been reports of fatalities from the biggest city, Yangon.

"Around six military vehicles were driving on a bridge, under which we were standing and demonstrating peacefully," Min Han Htet, president of a local student organization, told dpa.

"They were soldiers of the army, and they just shot at us," Min Han Htet said.

He said that several protesters were shot right in front of his eyes. "We thought they were shooting with rubber bullets but it was live ammunition," the 22-year-old said.

There were also deaths reported in the central city of Mandalay, as well as the nearby townships of Myingyan and Monywa, as the anti-coup protests showed no sign of abating on the 30th day of demonstrations since the military coup.

Four men and two women reportedly died in Myingyan, where one local journalist who talked to DPA said there were some 20,000 people on the streets.

Police broke up protests in several places in Yangon using rubber bullets and tear gas.

Protester Naing Naing Htet, 29, told DPA that 200 people had been arrested in Yangon. Some 5,000 people in total had been demonstrating, he said.

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"I am still studying at the university and we protest every day for justice. I was shot by two rubber bullets on [my] shoulder and on my back today and the neighboring people helped me," he said.

Two demonstrators, a 37-year-old man and a 19-year-old woman, died in the city of Mandalay, the newspaper MyanmarNow reported.

The US embassy in Yangon sharply condemned the crackdown, calling it "abhorrent."

"We are heartbroken to see the loss of so many lives in Myanmar. People should not face violence for expressing dissent to the military coup," the embassy wrote.

On Sunday, at least 18 people were killed and more than 30 wounded as the military crackdown on the protests continued, the UN Human Rights Office said. Graphic images of the wounded have flooded social media.

The UN Security Council has decided to discuss the crisis in Myanmar, most likely in the form of a closed session on Friday. The decision comes amid appeals from inside Myanmar for the UN to send aid. Sources on the ground are referring to it as a "war zone."