TOKYO -- Three bodies of people believed to be from North Korea 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 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 Saturday.

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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.

A Japan Coast Guard vessel sails near a drifting wooden boat carrying men identified as North Koreans off Matsumae on the northern Japanese island of Hokkaido, in this photo taken by Kyodo Nov. 29, 2017. Kyodo via Reuters

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

Twenty-eight of the vessels - dubbed "ghost ships" - were detected in November, up from just four in November last year. Usually, only the boats or fragments wash ashore. It is very rare for survivors to be rescued and brought ashore by the Japanese.

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 farther from their usual waters.

Japanese authorities are also holding 18 people from two other boats. They claim to be North Korean.

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 - 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.

Meanwhile, a large U.S.-South Korea air drill was held Monday following North Korea's test of its most powerful intercontinental ballistic missile last week. A North Korean newspaper called the joint exercise an "all-out provocation."

U.S. Republican Sen. Lindsey Graham told CBS' "Face the Nation" the Pentagon should take U.S. military families out of harm's way.

"It's crazy to send spouses and children to South Korea given the provocation of North Korea," Graham said. "So I want them to stop sending dependents, and I think it's now time to start moving American dependents out of South Korea."