MANDALAY, Myanmar — Monsoon downpours that set off flooding and a landslide in Myanmar late last week have killed at least 51 people and left dozens more missing, the authorities said, in the deadliest natural disaster to strike a part of the nation’s southeast in decades.

“This year’s flood is the worst in my life,” U Zaw Zaw Htoo, a member of Parliament from Paung Township in Mon State, which includes the area devastated by the landslide, said on Sunday.

Thirty inches of water drowned the township on Thursday and Friday, he said. The torrential rainfall in Mon State, a finger of land stretching down Myanmar’s coast, is expected to continue over the next couple of days.

Nearly 30 houses in Thaphyu Kone village in Mon State were completely buried by mud, officials said. And as he supervised rescue efforts in Thaphyu Kone, Ko Bo Bo Win, a community activist, said he worried that the crowds of onlookers who had gathered could be swept away by another landslide.

“It’s heartbreaking to see the dead bodies,” he added.

Ma Htay Htay, a resident of Thaphyu Kone, said she had just left for work on Friday when she heard a roaring noise behind her. Turning around, she saw her house disappear under mud.

Eight members of her family are missing, she said on Sunday, adding, “I’m praying hard that I will see them alive, but I don’t really have hope.”

More than 105,000 people across Myanmar have been displaced because of flooding since June, according to the United Nations Office for the Coordination of Humanitarian Affairs.