Heavy rains over the last several days caused the Yalu river, which marks the border, to breach its banks, although the water level had started to fall late Sunday, the official Xinhua News Agency state media said Monday.
It said four people died, including a couple in their 70s and a mother and son, after their homes in Dandong were swept away by flash floods. Xinhua said 253,500 residents have been evacuated after the Yalu rose to its highest level in a decade.
An official with the Water Resources Department in Liaoning province, where Dandong is located, confirmed that four people had died though he was unable to provide details. He refused to give his name because he was not authorized to speak with the media.
North Korea's official Korean Central News Agency said torrential rain and water from the overflowing Yalu — or Amnok as it is known in Korean — swamped houses, public buildings and farmland in more than five villages near Sinuiju, the city opposite Dandong.
The report described Sinuiju and the surrounding area as having been "severely affected" by the flooding and said officials, the military and ordinary civilians were involved in rescue work. It said at least 5,150 people had been evacuated and residents were clambering on rooftops or taking shelter on hilltops.
Much of North Korea's trade with the world passes through the city, forming a vital lifeline for the isolated, economically struggling country. Flooding in previous years has destroyed crops and pushed North Korea deeper into poverty, increasing its dependence on international food aid.
For China, the Dandong flooding is the latest disaster in the country's worst flood season in over a decade. Landslides caused by heavy rains have smothered communities in western China and accounted for most of the more than 2,500 people killed.
Authorities in the northwestern province of Gansu on Sunday called off rescue efforts for 330 people still missing after an Aug. 8 mudslide tore through Zhouqu county, killing 1,435 people, Xinhua said. The Zhouqu government forbade digging in the debris, fearing that recovering corpses buried for two weeks would spread disease.
Associated Press Writer Hyung-jin Kim in Seoul, South Korea, contributed to this report.