Official figures from the hardline communist state, quoted by relief agencies, earlier spoke of about 300 dead or missing.
But torrential rain, strong winds and landslides have doubled the total.
At least 600 are dead or missing and thousands injured, the official Korean Central News Agency (KCNA) said, citing figures from the Central Statistics Bureau.
The agency, in its starkest assessment so far of the damage, said downpours earlier this month "caused huge material losses to the (country), creating unprecedented difficulties in people's living and economic construction."
It said the homes of at least 240,000 families were totally or partially destroyed or inundated, leaving at least 100,000 people totally homeless and some 900,000 others flood-stricken.
Some 200,000 hectares of cropland was submerged, buried under silt or washed away just before the harvest, the agency said.
North Korea faced a food shortfall this year of one million tonnes, or 20 percent of its needs, even before the floods. It relied on international aid to help cover the shortfall.
KCNA said more than 8,000 public buildings, schools, health facilities and nurseries were totally or partly destroyed, along with more than 1,000 major production facilities since as mine complexes and factories.
It reported "serious damage to all sectors of the national economy."
The state-directed economy shrank an estimated 1.1 percent last year due partly to floods in 2006 and the international standoff over the North's nuclear programme, South Korea's central bank reported this month.
Relief agencies say this month's floods were the worst for a decade. The clearing and terracing of hillsides to create more cropland is the major reason for the periodic severe flooding.
The news agency reported severe damage to power plants and to coal mining, a crucial energy source.
It said dozens of substations were submerged and hydro-electric plants destroyed or inundated. Nearly 300 pits were flooded and hundreds of thousands of tons of coal were lost.
Some 100 stretches of road or railway line were washed away, and landslides buried rail track in at least 200 places, KCNA reported. More than 100 pumping stations for water or sewage were submerged.
"The army and people...are now turning out as one to recover from the damage with persevering will," it reported.
On Friday the UN's World Food Programme quoted the North's agriculture ministry as saying floods had hit 437,000 people and damaged more than one-fifth of the rice crop.
The WFP has announced an emergency programme to distribute food aid to 215,000 people for three months.
Separately, the UN's Office of Coordination of Humanitarian Assistance in New York announced it would launch an appeal for emergency funds of between 15 and 20 million dollars.
The International Federation of Red Cross and Red Crescent Societies has announced an appeal for 5.5 million dollars.
And South Korea Friday announced new aid worth almost 40 million dollars to help its neighbour rebuild. It has already begun trucking emergency food and other aid worth 7.5 million dollars across the heavily fortified frontier.
The National Defence Commission, chaired by state leader Kim Jong-Il, is supervising relief operations, state media said this week in a sign of the seriousness of the devastation.
