North Korean spies are feared to be hiding on "ghost ships" stranded off the coast of Japan.
The number of North Korean ships washing up on Japan's northwestern coast is mounting, with a record 28 running ashore in November alone.
Since the start of the year, 76 fishing vessels have ended up on Japanese shores. They have been labelled "ghost ships" because of the human skeletons found on board.
A ship that landed on the small Japanese island of Hokkaido in late November bore a sign that read "Korean People's Army 854 Military Corps."
Ten North Korean crew members were found on board. They claimed they were sheltering on the island from bad weather.
The Japanese media reported on Tuesday (5 November) that authorities found fishing equipment but no weapons on board.
A day before the discovery, eight decomposed bodies were found washed up in a small boat on a nearby beach. In the boat, lifejackets were found bearing Korean script.
The Japanese coast guard has tightened patrols in recent weeks amid fears that North Korean spies are sheltering on the stranded ships.
"The government is well aware that this is causing great anxiety to local people," Japan's Chief Cabinet Secretary Yoshihide Suga said.
"The police and coast guard... are working to clarify the situation," he said. "Once we have the facts, we will respond firmly."
Over the summer, the Japanese coast guard issued 820 warnings to North Korean ships that sailed into the Yamato Bank fishing grounds within Japan's economic zone, the Financial Times reported.
"What about the risk of these people, if they are special agents, making a landing just when some military operation is going on?" opposition politician Tetsuro Fukuyama said in Japan's parliament this week, referring to heightened tensions between the two countries as Pyongyang continues to advance its nuclear programme.
But Professor Ken Kotani of Nihon University told news.com.au that he did not think North Korea had launched a spy campaign, pointing to the fact that local authorities were handling the situation.
"The government has entrusted this to local police," he said.
Professor Satoru Miyamoto, from Seigakuin University, agreed that there was no reason for concern and said the rising number of ships was likely due to an increasing demand for fish.
"The number of wealthy people is growing and they're seeking a healthy diet. Fishing is meeting that demand," he said.