Nuclear tests in North Korea are causing deformities in newborn babies and are destroying the environment, defectors have claimed.

At least 21 escapees who spoke with the Research Association of Vision of North Korea said that recent tests in nuclear test site in Kilju, in the North Hamgyong Province, are contaminating the surrounding area with devastating consequences.

They claimed that 80% of the trees planted in Kilju have now died and the community fear possible water contamination due to radiation, South Korea's Chosun newspaper reported.

"I heard from a relative in Kilju that deformed babies were born in hospitals there," one defector said.

Another said: "I spoke on the phone with family members I left behind there and they told me that all of the underground wells dried up after the sixth nuclear test."

Defectors alleged they had not been informed before nuclear tests took place. They also said that before a drill is carried out, locals are ordered to dig holes in the ground for detonator tests, a procedure that can cause human casualties.

"Only family members of soldiers were evacuated to underground shafts. Ordinary people were completely unaware of the tests,"said one defector.

Another one claimed he saw "corpses floating down the river with their limbs severed".

Interviewees also alleged that people from Kilju are banned from travelling to the capital Pyongyang. They added that anyone caught boarding a train with sample of soil, water and leaves is arrested and "sent to prison camps".

The report emerged as US President Donald Trump is in South Korea, as part of his tour of Asia.

North Korea warned it would "mercilessly punish" the US if Trump does anything "reckless" while in South Korea.

Trump and North Korean leader Kim Jong-un have exchanged a series of fiery remarks and threats in recent months, exacerbating tensions in the region.