On Friday UN agencies said that half of the main health centres were submerged and that the situation could worsen unless aid arrives quickly.
"The situation is worsening as people are falling sick due to the poor hygiene conditions"
Jaap Timmer, International Federation of Red Cross and Red Crescent
The UN said at least 12 nations have offered or begun to provide aid to North Korea.
Jaap Timmer, head of the federation's delegation in North Korea, said they were trying to respond to the needs of at least 89,000 people who were homeless.
"But with clinics and hospitals put out of action and drug supplies ruined, the number of people whose lives have been impacted is far wider, and they need the international community's help," he said.
"The situation is worsening as people are falling sick due to the poor hygiene conditions."
The appeal aims to provide basic emergency treatment in the two worst-hit provinces, South Hamgyong and South Phyongan, and in the city of Kaesong, where most of the 3.7 million victims are.
|South Korean Red Cross sent the first shipment |
of supplies last week [EPA]
Anna Nelson, a federation spokeswoman, said that about half of the displaced families had no access to drinking water, with cases of diarrhoea up by 20 per cent.
The federation said the heaviest rains in 40 years over the past two weeks completely or partially destroyed 30 to 40 per cent of health facilities and supplies in North Korea, as well as homes, roads, bridges and crops.
There were also reports of an increase in acute respiratory infections, particularly in children, dehydration and skin and eye infections.
North Korea said 300 people were dead or missing, and another 300,000 left homeless, after torrential rains earlier this month led to flooding across the country, destroying thousands of hectares of farmland.
North Korea faced a food shortfall this year of one million tonnes, or 20 per cent of its needs, even before the floods.
The World Food Programme says it will start delivering emergency aid to feed 320,000 flood survivors once Pyongyang finalises its priorities.
It already operates feeding programmes for children and mothers.
Thousands of soldiers and civilians have begun clean-up efforts, according to official media.
Photographs showed hundreds of people shovelling mud off a road alongside the city's Taedong river.
The clearing of hillsides for farming and fuel is mainly to blame for the flooding.