North Korea has cut short summer military drills to mobilise troops for flood relief efforts after torrential rains left dozens killed and ravaged farmlands nationwide, according to a South Korean report.

The North's military ordered troops based in the country's west and southeast regions to hold "minimum" summer exercises and to instead focus on post-floods reconstruction, Yonhap news agency said.

It cited an unnamed Seoul government source.

"Many military units stopped the exercises and have mobilised troops for floods relief works," said the source quoted by Yonhap.

The communist state has staged summer military drills that partially coincided with the annual Ulchi Freedom Guardian exercise conducted by its rival South Korea and the United States, that usually takes place in August.

"But this year's summer drill in the North will be scaled back considerably because it needs to focus on repairing floods damages," the source was quoted as saying.

Floods caused by heavy rains that pummelled the North since early July have destroyed some 6,000 houses, displaced more than 23,000 people and washed away a large swathes of farmlands, the North's state media said late last month.

The death toll has reached 33 across the nation and some 13,300 hectares of farmlands have been damaged, the International Federation of the Red Cross and Red Crescent Societies (IFRC) said last week, warning of "longer-term impact" on the country's food security.

Decades of deforestation and decrepit infrastructure have left the impoverished North vulnerable to floods, which led to some 170 deaths last summer.