North Korean crewmen have admitted to stealing 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 over alleged theft after a TV, refrigerator and rice cooker went missing from the hut on the isle off Hokkaido. The shelter had been locked but was broken into and ransacked.

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

The North Koreans have told coast guard officers that they left the port of Chongjin in northeastern 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. One of the crew members was sent to the hospital on Monday after complaining of poor health, according to the sources.

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 the government was taking action.

"We are raising the alert level along the Sea of Japan coast areas with the coast guard's aircraft," he said. "We will take full measures to ensure security in territorial waters."

Wooden boats believed to be from North Korea had previously been found in Akita, Niigata and other prefectures along the Sea of Japan, totaling 28 cases in November, the highest figure in the last four years, according to the coast guard.