SEOUL, South Korea (AP) — Four suspected North Korean defectors were found in a small wooden boat near the two Koreas’ sea border Tuesday, South Korean officials said.

Defections by North Koreans have been a sore point in relations between the two Koreas. South Korea accepts those who choose to resettle in the South, but North Korea often says its people are held against their will in the South and demand they be returned.

A coast guard ship found the boat south of the sea border upon a report by a fishing boat and the four people on board identified themselves as North Koreans, coast guard officials said.

South Korea’s military said it secured the custody of the North Koreans in coordination with the coast guard, after chasing their boat along the two Koreas’ eastern sea border. The statement said the North Koreans were suspected of defecting to South Korea but gave no further details.

South Korea’s Unification Ministry confirmed an investigation of the North Koreans was underway but refused to provide details.

More than 30,000 North Koreans have fled to South Korea to avoid poverty and political oppression since the late 1990s. A vast majority of them have come via China and then Southeast Asian countries, and defecting by sea is uncommon.