Seoul urged Pyongyang on Wednesday to discuss the fate of hundreds of South Koreans said to be detained in North Korea, in response to its neighbour's repeated demand for the return of four defectors.
The South's Red Cross offered to hold talks with its Northern counterpart on May 4 at the border truce village of Panmunjom, said Seoul's unification ministry which handles cross-border affairs.
"Since the North insists the four North Koreans are being held in the South against their will, we are proposing talks to verify the free will of our own nationals held in the North," a ministry spokeswoman told AFP.
The four North Koreans were in a group of 31 whose boat drifted across the border in thick fog in February. Seoul says four of them chose to stay in the South while 27 others were sent back in March.
The communist North initially refused to accept the return of the 27, saying all 31 must be sent back, but later relented.
But it repeatedly called for Seoul to return the four, saying they were brainwashed and pressurised by the capitalist South.
Next week's talks, if they go ahead, would also discuss ways for the North to verify that the four chose to stay of their free will, the ministry said.
Seoul says around 500 of its people, mostly fishermen, were seized in the decades following the 1950-53 war and more than 500 prisoners of war were never sent home in 1953.
Pyongyang says it is holding no one against their wishes.
Cross-border relations have been icy following what Seoul says were two deadly attacks by Pyongyang in 2010 near the disputed Yellow Sea border.
South Korea's navy fired warning shots to drive away a North Korean boat that briefly violated the border on Tuesday, military officials said on Wednesday.
"The North Korean boat sailed back into its territory after staying in our waters for 24 minutes," an official with the joint chiefs of staff told AFP.
"We believe the boat violated the border while following Chinese fishing boats."