A day after North Korea suspended communication hotlines with South Korea over defectors who send propaganda and contraband into the North, South Korea said it would take legal action against two organisations that conduct such operations.
North Korea gets enraged when the defectors in the South send material such as anti-North leaflets and rice - usually by balloon over the heavily fortified border or in bottles by sea - and its media has in recent days denounced the "mongrel dogs" who do it.
Kim Yo Jong, the sister of North Korean leader Kim Jong Un, recently called defectors "human scum little short of wild animals" and said North Korea would cut communication with South Korea because of its failure to stop them.
South Korea, which is trying to improve ties with the North, said on Wednesday two defector-run groups, Kuensaem Education Center and Fighters for a Free North Korea, had violated the Inter-Korean Exchange and Co-operation Act by sending the leaflets, as well as aid like rice and medicine.
The two defector groups "have created tension between the two Koreas and caused danger to the border-area residents' lives and safety", said the South's Unification Ministry spokesman Yoh Sang-key.
One defector, Park Sang-hak, who left North Korea in 2000 and heads the Fighters For Free North Korea, has been sending leaflets about once a month for the last 15 years.
"You can never buy peace with flattery and begging," he said of the South Korean government's response to the North Korean criticism.
About 33,000 North Korean defectors live in South Korea.
As part of the effort to improve ties with the North, South Korean President Moon Jae-in's administration has sought to discourage the leaflet and rice campaigns, and defectors complained of pressure to avoid criticism of North Korea.
On Monday, activists were stopped by residents when they tried to send plastic bottles stuffed with rice by releasing them at sea.