SEOUL (REUTERS) - A North Korean defector-led group on Thursday (June 18) prepared hundreds of plastic bottles stuffed with rice which it plans to float into North Korea, despite a legal challenge from South Korean authorities and threats from Pyongyang.

Citing South Korea's failure to stop the defectors, North Korea this week blew up the joint liaison office on its side of the border, declared an end to dialogue with South Korea and threatened military action.

Denouncing defectors as "mongrel dogs" and "human scum", North Korea says their activities are an insult to the dignity of the country's supreme leader.

South Korea is keen to improve relations with North Korea, and last week the government announced it would pursue legal actions against two defector-led groups, saying their cross-border shipments of aid and propaganda were raising tensions with North Korea, posing risks to South Koreans who live on the border, and causing environmental damage.

Still, one group is planning to send hundreds of bottles stuffed with rice, medicine and medical face masks to North Korea by throwing them into the sea near the border on Sunday, said Mr Park Jung-oh, 61, a North Korean defector who heads the group, called Kuensaem.

"We do this as humanitarian aid among those who share the same values, so whatever North Korea says, we will continue to help those in hard situations, the elderly and the victims," he said.

Gathered at a small park in Seoul, the group filled dozens of 2 litre bottles with up to 1.5kg of rice each, sending up to 700kg of rice in total, Mr Park said.

Kuensaem has been sending goods to the North twice a month for the past five years. Sunday will mark their 108th time, Mr Park said.

The South Korean authorities have occasionally moved to stop such operations, including in 2018 during a series of summits between South Korean President Moon Jae-in and North Korean leader Kim Jong Un.

"I don't know why the Unification Ministry is nitpicking at us all of a sudden. The (South Korean) government - the Gangwha police, maritime police and the military - all knew about us,"said Mr Park, unloading rice and bottles from his white truck.

He said he had not been contacted by any South Korean authorities since they announced they would pursue legal action.

Several defector-led groups regularly send fliers over the border, together with food, US$1 bills, mini radios and USB sticks containing South Korean dramas and news. Most use balloons or bottles in rivers.

The two Koreas traded fire in 2014 after the North's military fired machine guns at balloons launched by defector activists.