Switzerland is renewing direct talks with North Korea as tensions rise on the peninsula. Seoul hopes the Swiss office will help it engage with Pyongyang.
Missile launches and a dispute over a landmine incident have heightened tensions between the two Koreas. Seoul is seeking military talks to prevent accidental clashes, but Pyongyang has yet to respond.
The reported test comes after North Korea tested two types of underwater-launched strategic cruise missiles on Wednesday, in what is believed to be a push to bolster its naval capabilities.
Japan's military said the missile landed in waters outside its exclusive economic zone, while South Korea's military, which often accuses the North of exaggerating its capabilities, said it travelled more than 700 kilometres.
North Korea in recent years has added mines, anti-tank barriers and other front-line fortifications, raising concerns in the South about the growing presence of North Korean soldiers near the border.