Photo/Illutration A Russia-flagged fishing boat which was detained by North Korea, is seen at a port in Sokcho, South Korea on July 28. (Yonhap via AP)

SEOUL--Two South Koreans and 15 Russians returned to South Korea on Sunday, about 10 days after their boat drifted into North Korean waters, Seoul officials said.

The crew members were aboard a Russia-flagged fishing boat when it was detained by North Korea on July 17. The ship had been on its way to Russia after leaving South Korea's eastern Sokcho port.

Seoul's Unification Ministry said in a statement the crew arrived aboard the same boat at Sokcho port on Sunday, a day after they left the North's Wonsan port. Details of how they were detained, treated and repatriated were still unclear as the ministry said 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.

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.