(CNN) -- North Korea Friday unilaterally informed South Korea that all contracts relating to a jointly-run industrial complex along their border are null and void, according to South Korean officials.
The North also said that if the South does not accept the North's unilateral demands, South Korean companies can leave the Kaesong industrial complex, north of the demilitarized zone, said Kim Ho-Nyun, the South's Unification Ministry spokesman.
"The South Korean government is expressing deep regret at this action. And the South cannot accept such a unilateral and basic threat," Kim said.
The North accused the South Korea of being "insincere" in negotiations and suggested that Pyongyang might shut down the complex, according to South Korea's Yonhap news agency.
"We believe that such words are irresponsible and hurts the basic stability of Kaesong," Kim said. "We also believe that the rights of the companies that are in Kaesong must be acknowledged.
"We have suggested that South and North Korean officials sit down for talks on May 18," the spokesman said.
The companies at the Kaesong complex have not responded officially.
North Korea has not stopped workers from entering or leaving the complex.
In March, it temporarily closed its border with the South, stranding hundreds of Kaesong workers. At the time, the North said the closing was in response to annual joint military exercises by South Korea and the United States, which it called a threat to its security.