The death toll of last week’s deadly floods and landslides in the Democratic Republic of Congo climbed to more than 400 people, the government said on Monday, as rescue workers and family members searched through debris and mud for victims and survivors of the disaster.

The flooding began on Thursday, as heavy rains pushed rivers to overflow their banks, causing rushing water and mud to flow into villages, washing away homes and ravaging farmland.

“The situation is catastrophic,” said Remy Kasindi, a representative for a civil society group in South Kivu, where the floods took place. “It’s a humanitarian crisis that troubles our consciences.”

Mr. Kasindi said that survivors were looking for their family members in the mud, and that some bodies were pulled from nearby Lake Kivu.

Nearly 3,000 families were left homeless by floods and landslides, with their houses damaged or destroyed, the U.N. Office for the Coordination of Humanitarian Affairs said. Twelve hundred homes were completely razed, the office said. On Monday, Congo observed a day of national mourning for the victims of the flooding.

Before the flood, people from the region had traveled to the area to sell their agricultural products at a market there, making in harder to count the total number of the missing, rescuers said. On Monday, bodies were still being pulled out of the rubble.

Several news media outlets reported that the authorities had to bury the bodies in mass graves.

Dr. Denis Mukwege, a Congolese winner of the Nobel Peace Prize in 2018, pleaded on Twitter for the dead to be treated with dignity.

“Exhume the bodies, identify them by DNA, bury them individually and not in a mass grave,” he wrote.

The floods hit the Kalehe Territory in South Kivu, and specifically the villages of Bushushu and Nyamukubi. The area affected by the floods had welcomed thousands of displaced people from the province of Nord-Kivu, which has been plagued by fighting between the Congolese Army and rebel forces.

While the dry season for this region is fast approaching, the next 10 days look likely to have more rainfall, some of which could be higher than normal. Any rain, above average or not, that falls on the already saturated ground could increase the risk of flooding because the land and rivers already have abundant water and the rainfall has nowhere to go. The area is also mountainous, increasing the risk of landslides.

The villages are next to the shore of Lake Kivu, which separates Congo from Rwanda. Last week, more than 120 people were killed in devastating floods and landslides caused by heavy rains in Western Rwanda.

On Saturday, during a visit to Burundi, the United Nations secretary general, António Guterres, said that the floods were “yet another illustration of accelerating climate change and its disastrous impact on countries that have done nothing to contribute to global warming.”

Experts have said that while linking specific events to climate change requires extensive analysis, climate change has made extreme rainfalls more frequent and intense.

Last year, more than 140 people were killed by floods and landslides in Congo’s capital, Kinshasa, and Central Africa as a region has recently been afflicted by deadly floods.

Local and international charities have provided help to the area, trying to bring food or health assistance to prevent the spread of waterborne disease. Cholera is endemic in the area, the United Nations said.

Doctors Without Borders strengthened its presence to help the wounded, and the Red Cross was deployed in the search of bodies and to help with the organization of the burials.

Judson Jones contributed reporting.