According to the aid agencies, the North Korean government has reported 133 deaths with nearly 400 people missing and homes and crops destroyed.
Rescue teams have been unable to reach some of the worst-hit areas.
North Korea already has chronic food shortages and is heavily dependent on foreign aid to feed its population.
The United Nations has allocated $8 million in 2016 for humanitarian aid in North Korea.
The flooding, triggered by the recent Typhoon Lionrock, comes as North Korea faces global anger for conducting its fifth nuclear test last week.
September 9 detonation, believed to be the North’s biggest test so far, is expected to lead to a tightening of sanctions.
According to the UN, the worst flooding is along the Tumen River, which borders China. Many areas in Musan and Yonsa counties are entirely cut off.
The UN’s Office for the Co-ordination of Humanitarian Affairs (OCHA) has said 140,000 people are “in urgent need of assistance”.
Murat Sahin, a UN official in North Korea, said the scale was of the disaster was “beyond anything experienced by local officials”.
According to North Korean state media, people are experiencing “great suffering” in the region.
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