The North Korean crew aboard a recently impounded vessel have admitted to stealing home appliances from a refuge hut on an uninhabited islet in northern Japan where they took shelter due to bad weather, investigative sources said Tuesday.

Police and the Japan Coast Guard have questioned the fishermen on suspicion of theft after a TV, refrigerator and rice cooker went missing from the hut on the isle off Hokkaido, the northernmost of Japan's main islands. The hut had been locked but was broken into and ransacked.

When the coast guard found the wooden fishing boat with 10 North Korean men aboard on Wednesday, they were seen dumping some appliances and other items into the sea, investigative sources said, adding the coast guard collected some of the objects later.

The North Koreans have told coast guard officers that they left the port of Chongjin in the northeast of North Korea in September for squid fishing in the Sea of Japan before their steering wheel failed around a month ago.

The coast guard towed the wooden boat into waters off Hakodate port on Sunday and the crew remain aboard. One of the crew was sent to hospital on Monday after complaining of poor health, according to the sources.

A plate hanging on the boat suggests it belongs to North Korea's military and the Hokkaido police are continuing to investigate where the crew came from and other details.

The plate reads "Korean People's Army No. 854 military unit," according to the sources. In North Korea, military members also engage in farming and fishing.

The local police also found squid and fish nets on the boat, but no weapons, the sources said.

In the sea off Niigata Prefecture, two wooden boats were seen drifting and two bodies were found Tuesday, the local coast guard said, adding they are investigating whether the vessels also came from North Korea.

Following a slew of wooden boats from North Korea drifting ashore on the Sea of Japan coast, Keiichi Ishii, minister of land, infrastructure, transport and tourism said Tuesday, "We are raising the alert level along Sea of Japan coastal areas...We will take full measures to ensure security in territorial waters."

In November, there were 28 cases of wooden boats apparently from North Korea being found in Akita, Niigata and other prefectures along the Sea of Japan, the highest figure in the last four years, according to the coast guard.