Some 6.6 million people in Nepal have been affected by the devastating earthquake, the UN office in Kathmandu said on Sunday.
The quake triggered an avalanche in the Mount Everest area, burying parts of the Everest base camp.
"Rescue and relief operations may be hampered with the monsoon fast approaching," said the UN.
The United Nations Resident Coordinator's Office and other UN agencies will work under the leadership of the Nepal government with the support of a United Nations Disaster Assessment Team.
The team arrived in Kathmandu on Sunday to assist in identifying the most immediate need of those affected by Saturday's quake.
The UN Resident Coordinator, Jamie McGoldrick, said: "We are ready to assist the government of Nepal to respond to this terrible tragedy.
"It is essential that we move quickly and effectively," he said. "We need to ensure that no further lives are lost and the needs of the most vulnerable prioritized."