North Korean state media says nearly 170 people have been killed by widespread flooding.
The fate of about 400 more is still not known.
More than 212,000 are homeless.
Video provided by KCNA news agency showed flooded streets in Anju City about 67 kilometres north of Pyongyang.
Garment workers trapped on the upper floor of their factory said the water rose very quickly.
(SOUNDBITE) (Korean) YUN YONG-SUK, WORKER OF ANJU GARMENT FACTORY, SAYING:
"The water was flooded in like a rising tide in a few seconds, so we couldn't remove our materials in time and the place was submerged before we could move our sewing machines out."
Local security official says the city's river is well above the danger level.
(SOUNDBITE) (Korean) SUNG CHANG-JONG, CHIEF OF SECURITY STATION OF ANJU CITY, SAYING:
"The height of the Chongchon river bank is 11 metres, but currently the water level is at nine metres. The danger level is actually seven metres, so we are above that now."
South Korea's unification ministry says the government has no plan to send aid to the North.
It sent flood aid in 2010, but last year Seoul said the North did not respond to its offer.
