A heatwave in North Korea has led to rice, maize and other crops withering in the fields, "with potentially catastrophic effects", the International Federation of Red Cross and Red Crescent Societies said.

The world's largest disaster relief network warned of a risk of a "full-blown food security crisis" in the isolated country hit by international sanctions.

The population is already stressed and vulnerable with malnutrition among children that could worsen, it said.

There had been no rainfall since early July as temperatures soared to an average 39C (102F) across the country with the next rain was expected sometime this month.

The population of 25 million is already stressed and vulnerable with malnutrition among children that could worsen, the network said in a statement issued in Geneva.

"This is not yet classified as a drought, but rice, maize and other crops are already withering in the fields, with potentially catastrophic effects for the people of DPRK," said Joseph Muyamboit, its programme manager in Pyongyang.

"We cannot and must not let this situation become a full-blown food security crisis. We know that previous serious dry spells have disrupted the food supply to a point where it has caused serious health problems and malnutrition across the country," he said.

The federation was helping the national Red Cross to support 13,700 of the most vulnerable people at risk. It had deployed emergency response teams and 20 water pumps to irrigate fields in the hardest-hit areas.

David Beasley, the head of the UN's World Food Programme (WFP), visited North Korea in May to look into boosting food distributions to hungry women and children, in the latest sign of an opening.

About 70% of North Koreans are "food insecure", meaning they struggle to avoid hunger, and one in four children under five is stunted from chronic malnutrition, the WFP said at the time.

A 2015 drought worsened the situation, it said.

North Korea suffered famine in the mid-1990s that killed up to three million people.