SEOUL—North Korea on Sunday seized a South Korean fishing boat and its seven-person crew in waters off the North's east coast, but the exact location and circumstances weren't known, the South's coast guard said.

Such incidents occur periodically between the two Koreas, which are bordered on multiple sides by water and have sizable fishing industries. Last year, North Korea detained a South Korean fishing vessel crew for a month for allegedly straying into northern-controlled waters.

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