The head of neighboring India's National Disaster Response Force (NDRF), one of the first foreign organisations to arrive in Nepal to help in the search and rescue effort, said finding survivors and the bodies of the dead would take time.
NDRF Director General O.P. Singh said heavy equipment could not fit through many of the narrow streets of Kathmandu.
"You have to remove all this rubble, so that will take a lot of time ... I think it's going to take weeks," he told Indian television channel NDTV late on Monday.
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Many people across Nepal slept in the open for a third night, their homes either flattened or threatened by tremors that spread more fear among a traumatized population.
In Kathmandu, as elsewhere, thousands are sleeping on pavements, roads and in parks, many under makeshift tents.
Hospitals are full to overflowing, while water, food and power are scarce, raising fears of waterborne diseases.
With aid slow to reach many of the most vulnerable, some Nepalis were critical of the government.
"The government has not done anything for us," said Anil Giri, who was with about 20 volunteers looking for two of his friends presumed buried under rubble. "We are clearing the debris ourselves with our bare hands."
Officials acknowledged they were overwhelmed by the scale of the disaster.
"The big challenge is relief," said Chief Secretary Leela Mani Paudel, Nepal's top bureaucrat.
"We urge foreign countries to give us special relief materials and medical teams. We are really desperate for more foreign expertise to pull through this crisis."
"My duty to find her"
The situation is worse in remote rural areas. Highways have been blocked by landslides, and many villages and communities are without water and electricity, surviving on salvaged food and with no outside help.
While aid has begun arriving in the capital, including food, medical supplies, tents and dogs trained for rescue efforts, the authorities are struggling to deliver relief further afield.
Time is running out to find survivors among the rubble of collapsed buildings. In the north of Kathmandu, survivors suspended their search overnight, with plans to resume early on Tuesday.
"We cannot look for missing people ... with a candle in our hands," said Amarnath Prasad, 26, a musician who was helping his best friend look for his missing mother.
"She loved me like her son, and I think it is my duty to find her, dead or alive," said Prasad.
Families and friends of the victims lit hundreds of funeral pyres in towns and the countryside.
Foreign countries escalated efforts to get medical equipment, medicine, food, water, blankets, tents and search-and-rescue teams into Nepal, but chaotic scenes at the main international airport slowed the flow of aid.
India and China were among the first contributors to an international effort to support Nepal's stretched resources.