China has suspended traffic on the Yalu river, which marks the border with North Korea, because of record rainfall in an area already badly hit by floods.
More than 40,000 people have been moved from their homes in the Chinese border city of Dandong.
Red Cross workers in North Korea have reported heavy damage by floods in the east of the country.
They say buildings, bridges and roads have been destroyed, but warnings had enabled many to move to higher ground.
The Yalu river in north-east China has swollen to critical levels after two weeks of unprecedented rainfall, Chinese state media reported.
It has led to fears of mass flooding in North Korea, which, according to its official news agency, suffered heavy flood damage in July with more than 5,000 houses destroyed.
The Korean Central News Agency said 360 buildings, including clinics, bridges and factories, had also been destroyed and huge swathes of farmland rendered useless by flash floods.
There have been no reports of serious injuries or fatalities, but such information is difficult to verify outside the reclusive state.
A team of Red Cross workers in North Korea's South Hamgyong province told the BBC that they had witnessed widespread destruction.
Doctors told the Red Cross that a number of people, especially the elderly, were suffering from shock and had "clear symptoms of traumatic stress reactions".
The Red Cross had also heard reports from South Phyongan province that the water system was severely damaged by a reported landslide, leaving an entire city without clean drinking water.
Particularly heavy summer rains in China's north-east have triggered the country's worst flooding in a decade and left nearly 1,700 people dead or missing.
More rain was expected in the region on Friday, according to a spokesman for the flood control headquarters in Dandong, quoted by Xinhua news agency.