Three bodies of people believed to be North Koreans were recovered in northern Japan on Monday, two days after authorities found a dilapidated empty boat, coast guard officials said.

The coast guard said a Japanese fishing boat picked up a male body floating off the coast of Sakata in Yamagata prefecture and two more bodies washed up on a nearby beach an hour and a half later. The bodies were decomposed, but one had a lapel pin thought to be North Korean.

Officials are investigating if the bodies were from the severely damaged boat that washed ashore on Saturday.

Winds and water currents push dozens of boats onto Japan's northern coasts annually. Rickety North Korean fishing boats are particularly vulnerable because they lack the sturdiness and equipment to return home.

But the alarming pace over the past few weeks has prompted Japanese authorities to step up patrols.

Twenty-eight of the vessels — dubbed "ghost boats" — were detected in November, up from just four in November last year. Usually, only the boats, sometimes with bodies inside, or fragments wash ashore. It is rare for survivors to be rescued and brought ashore. Coast guard officials have recovered at least 18 bodies this year.

Inside the daily life in North Korea Show all 19 1 /19 Inside the daily life in North Korea Inside the daily life in North Korea People reading a newspaper at the metro station Inside the daily life in North Korea Thoughts of the leaders on the tram. They have about a dozen of these on every tram, all with different thoughts Inside the daily life in North Korea Young people training for a big upcoming festival Inside the daily life in North Korea People at the Pyongyang's annual marathon Inside the daily life in North Korea Many stars on one of the trolleys in Pyongyang Inside the daily life in North Korea An intimidating poster in a primary school in North Korea. Inside the daily life in North Korea Solar panels installed on a street lamp. Inside the daily life in North Korea A poster on the window next to one of the venues we visited in Pyongyang Inside the daily life in North Korea Kids playing football next to the Arch of Triumph. After a while tourists were allowed to join, so some of us did Inside the daily life in North Korea Class in an educational center in Pyongyang (where people over 17 years old can attend any classes they choose after school, for free) Inside the daily life in North Korea People waving at me during the Pyongyang marathon Inside the daily life in North Korea People having a great time dancing at a public park Inside the daily life in North Korea A metro driver in a metro station in Pyongyang Inside the daily life in North Korea Fireworks to mark the birthday of the Eternal President Kim Il Sung on our last night in Pyongyang Inside the daily life in North Korea My wonderful tour guide at a public park Inside the daily life in North Korea One of the parks in Pyongyang Inside the daily life in North Korea A person rowing some boats for the day at a river in Pyongyang Inside the daily life in North Korea The National War Museum Inside the daily life in North Korea Public park in Pyongyang

The increase may be related to a campaign pushed by leader Kim Jong-un to boost fish harvests as a means of increasing sources of protein for the nation, which continues to fall short of food self-sufficiency and remains vulnerable to health problems caused by the lack of a varied, balanced diet.

In order to reach their quotas, the North Korean fishermen may be taking more risks and venturing further from their usual waters, approaching or possibly violating Japan's 200-nautical mile exclusive economic zone west of Japanese northern coasts, known for rich fishing grounds.

Japanese authorities are also holding 18 people from two other boats. They claim to be North Koreans and reportedly have expressed a desire to return home.

The first batch of 10 landed on a small uninhabited island off southern Hokkaido on a damaged fishing boat and allegedly stole electronic appliances and other items from an unmanned shelter while temporarily taking refuge from rough seas. Japan's coast guard rescued them last week.

Eight other survivors managed to reach shore in Akita on a ragged boat — which is believed to have fallen apart and sunk soon after they were rescued — and have been transferred to immigration custody.

Japanese officials said the 10 are being investigated for possible theft, while the other eight are expected to be sent home via China.