Some 107,000 people have been forced to flee their homes in the area along the Tumen River, the world body said in a statement received today which cited Pyongyang government figures.
The North's official media has described the downpour which led to the floods near the northeastern border with China and Russia as the worst for decades, and said it brought severe hardship to residents.
The impoverished nation is vulnerable to natural disasters, especially floods, due partly to deforestation and poor infrastructure.
At least 169 people were killed by a massive rainstorm in the summer of 2012.
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Major state resources are swallowed up by a missile and nuclear weapons programme which Pyongyang says is essential to deter US aggression.
More than 35,500 houses have been hit by the latest floods, with 69 percent of them completely destroyed, and 8,700 public buildings have been damaged, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement dated Sunday.
OCHA said a group made up of UN agencies, international NGOs, the international Red Cross and the North's Red Cross had visited parts of the flood-stricken region last week to assess needs.
It said aid agencies have released material from stockpiles in the North such as food, shelter and kitchen kits and water purification and health supplies.
The North's government was working urgently to reopen roads and was distributing relief goods and building materials. The priority was to rebuild 20,000 homes by early October, before the bitter Korean winter sets in.