The Red Cross appealed Wednesday for $15.5 million in emergency funding to help flood-ravaged North Koreans, warning of a "secondary disaster" in the impoverished country unless urgent assistance is provided.

At least 138 people are known to have died and nearly 400 are missing after torrential rain triggered major floods, devastating villages in the country's northeast, the UN said last week.

According to the UN, 140,000 people need assistance. The International Federation of Red Cross and Red Crescent Societies (IFRC) said that about 70,000 remain homeless after tens of thousands of houses were damaged or destroyed.

"People were vulnerable before this disaster but now they are in danger of reaching tipping point," said Chris Staines, head of the IFRC delegation in Pyongyang, in a statement.

"Winter is on our doorstep and when you add up the impact of the floods and the risks people now face, we could see a secondary disaster here in the months ahead", Staines said, following a visit to the affected areas.

With winter temperatures threatening to plunge to minus 30 degrees Celsius (minus 22 degrees Fahrenheit), the $15.5-million fund will be used to deliver crucial relief supplies including tents, medicines and coal to 7,000 families.

Images published by state media showed collapsed roads and railways, as well as houses engulfed in a wall of mud, while hundreds of people -- many covered in dirt -- struggled to remove the debris.

It said large numbers of troops and residents from nearby areas had been mobilised for reconstruction, with authorities urging them to "work miracles".

The impoverished and isolated North is vulnerable to natural disasters, especially floods, due partly to deforestation and poor infrastructure.

At least 169 people were killed by a massive rainstorm in the summer of 2012.

However, huge government resources are swallowed up by a missile and nuclear weapons programme widely condemned by the international community.

The North, already under layers of sanctions imposed over past missile and nuclear tests, is facing potentially more sanctions following a fifth nuclear test staged on September 9.