Severe flooding in North Korea has destroyed more than one-tenth of the country's farmland, according to the state news agency KCNA.
"As of 14 August, more than 11% of rice and maize fields were submerged, buried or washed away," Ri Jae-Hyon, director of the Ministry of Agriculture, said.
Government officials also told aid workers in the region that 300,000 people may have been left homeless.
Aid teams visiting the area warned of a need for emergency shelter and food.
"Going forward, the crop damage is of major concern," Michael Dunford, of the UN World Food Programme, told the BBC.
He added that he had been to see some of the damaged areas, and described the situation as "pretty grim".
"Areas of the capital, Pyongyang, have been inundated," he said, "but certainly as you move out into the countryside there is widespread damage, and it is going to have a negative impact on DPRK [North Korea] most certainly."
North Korea already suffered from severe food shortages, even before the floods.
The flood has destroyed roads and bridges, KCNA said
About two million people are thought to have died from famine in the mid-1990s in North Korea, and the country remains dependent on foreign food aid.
United Nations Secretary General Ban Ki-moon has ordered a full evaluation of the needs of North Koreans and has promised assistance to the communist nation.
"I assured him that the United Nations will be prepared to render whatever possible humanitarian assistance and help to the DPRK (North Korean) government and people overcoming this difficulty," he said after a meeting with North Korea's UN envoy Pak Gil-yon.
The US and South Korea have both said that they would consider sending aid.
North Korea made the rare plea for help after it announced late on Monday that storms since 7 August had led to "huge human and material damage".
State news agency KCNA said hundreds of people were dead or missing.
Many areas were affected but worst hit were the three provinces of Kangwon, North Hwanghae and South Hamgyong, it said.
Television pictures from the capital Pyongyang showed people wading along streets through thigh-deep water after rivers burst their banks.
These floods are thought to be worse than the ones that hit last year. Hundreds of people are thought to have died in August 2006, but exact figures are not known.