Two South Koreans and 15 Russians returned to South Korea on Sunday, following 10 days of detention in North Korea after their fishing boat drifted into North Korean waters, officials said.

The crew members were aboard a Russia-flagged fishing boat when it was detained by North Korea on July 17 after leaving South Korea’s eastern Sokcho port a day earlier.

Seoul’s Unification Ministry said in a statement the crew arrived aboard the same boat at Sokcho on Sunday, a day after they left the North’s Wonsan port.

Seoul officials did not immediately explain how they were detained, treated and repatriated, saying North Korea hasn’t informed South Korea of its decision to release the crew. The ministry said it learned of the boat’s departure from Wonsan on Saturday through various channels that it refused to disclose.

The ministry statement said it “positively” assessed the North’s repatriation of the crew members.

In a Facebook message, the Russian Embassy in Pyongyang said the “coordinated work of the Russian diplomatic agency” led to their release. It said the ship’s captain was informed of the crew’s planned release 3 ½ hours before their departure from Wonsan.

An unidentified crew member told Russia’s state news agency Tass that the ship was released without being fined. “The full crew is aboard the ship, everyone is feeling well,” he was quoted as saying.

The Tass report said the ship was heading to the waters between the Korean Peninsula and Japan to catch crabs when it was detained. It cited the North Korean foreign ministry representatives as saying the ship was detained for “violating the rules of entry and stay in North Korea.”

Ties between the Koreas remain cool amid a lack of progress in U.S.-led diplomacy aimed at ending North Korea’s nuclear program.

Seoul said North Korea is holding six other South Koreans it has arrested in recent years on anti-state and other charges.

Fishing boats drift across the Koreas’ eastern sea border in both directions. Earlier Sunday, South Korea’s military said a North Korean wooden fishing boat carrying three people crossed the maritime border on Saturday night, prompting a South Korean navy ship to tow it to a South Korean port.

South Korea’s Joint Chiefs of Staff said the North Koreans were under an investigation. South Korea typically returns North Korean fishermen unless they are suspected of espionage. But it also lets them resettle in the South if they want, often triggering angry response from the North.

(AP)