At least 38 people were killed in Myanmar on Wednesday, the biggest one-day toll in a worsening repression of anti-coup protests, the United Nations special representative for the country said.

The representative, Christine Schraner Burgener, reported the deaths as news emerged that the junta’s own choice for U.N. envoy had abruptly resigned.

Wednesday’s developments came as the United States, which is president of the United Nations Security Council for March, scheduled a meeting on Friday to deal with the crisis in Myanmar, diplomats said.

Myanmar has been thrown into turmoil since a military junta seized control on Feb. 1 and arrested the civilian leaders whose party, the National League for Democracy, had won an overwhelming victory in national elections. Security forces have used increasingly brutal means to crush the anti-coup protests.

Ms. Burgener, a Swiss diplomat appointed by Secretary General António Guterres three years ago to monitor Myanmar, told reporters at a news conference that junta leaders had rejected her requests to visit the country.

When she warned them of the consequences of becoming an international pariah, Ms. Burgener said that they responded: “‘We are used to sanctions,’” and “‘We must learn to walk with only a few friends.’”

Ms. Burgener also said she had received many messages from Myanmar citizens inside the country pleading for international action that would end the repression and lead to the release of the arrested civilian leadership headed by Daw Aung San Suu Kyi, the Nobel laureate.

“Today was the bloodiest day since the coup happened,” she said. “Only today, 38 people died.”

Ms. Burgener did not specify the sources of her information or elaborate on where in Myanmar the deaths had occurred. But other news accounts and social media postings from Myanmar reported similar figures from clashes in several cities. The number of deaths, if confirmed, would be roughly double the previous one-day record of 18 killed on Sunday.

She spoke as intrigue deepened at the United Nations over Myanmar’s diplomatic representation at the 193-member organization.

The machinations began when U Kyaw Moe Tun, the ambassador, denounced the junta leaders on the global stage last Friday in an emotional speech that made him an instant hero among many fellow diplomats and pro-democracy activists at home and elsewhere.

On Saturday, the generals said he had been fired. On Tuesday, United Nations officials said Mr. Kyaw Moe Tun had declared in writing he remains the rightful representative of Myanmar. But they also said Myanmar’s Foreign Ministry had declared in writing that Mr. Kyaw Moe Tun was no longer ambassador, and that his deputy, U Tin Maung Naing, was acting as Myanmar’s top U.N. diplomat.

On Wednesday, Mr. Tin Maung Naing announced via Facebook that he had resigned, with no explanation.

Stéphane Dujarric, a spokesman for Mr. Guterres, said he was aware of the resignation but said there had been “no official communication” about it. He also said that the conflicting information from Myanmar had been shared with the United Nations General Assembly’s Credentials Committee, which deals with disputes that arise about diplomatic representation at the United Nations.