The International Federation of Red Cross and Red Crescent Societies (IFRC), the world's largest humanitarian network, fears the death toll from Wednesday's disaster, already above 500, will continue to rise.
"Due to roads and bridges having been washed out and both cell coverage and electricity cuts, we have not yet been able to access some of the hardest-hit areas," said David Fisher, the IFRC head of delegation for Nepal.
"The trauma of this event is enormous," he told a press conference in Geneva, speaking from Kathmandu.
"Thousands of homes have been destroyed or suffered significant damage. We believe that at least 93,000 people have been affected and in great need.
"The death toll is very high and unfortunately likely to continue to rise."
The Nepal Red Cross branch has begun providing emergency relief for those who have lost their homes, including food, water, hygiene kits, mattresses and blankets.
The floods and landslides that struck Nepal and China's autonomous Tibet region saw a tsunami-like wall of water and mud come crashing down on Wednesday, burying homes, smashing bridges and sweeping away vehicles.
UN aid chief Tom Fletcher said on X that the United Nations was "moving at speed to save lives".
Lila Pieters Yahia, the UN resident coordinator for Nepal, said drinking water was being airlifted into affected areas.
"The damage is just incredible," she told the press conference, speaking from the field, adding that more than 40 bridges had been destroyed or badly damaged and "over 40 kilometres of road completely destroyed".
She said search and rescue operations remained the government's immediate priority.
The World Health Organization said eight health workers were missing and six health facilities had been damaged.
"Right now, really the priority is to ensure continuity of health services," WHO spokesman Tarik Jasarevic said.
He said four emergency medical teams had been deployed from Kathmandu to the affected districts, with others on standby.
He said the UN health agency had delivered surgical masks and gloves and body bags to Nepal's health ministry.