SEOUL, South Korea  North Korea seized a South Korean squidding boat in waters near their eastern sea border, the South Korean Coast Guard said Sunday, straining already high tensions between the two Koreas.

The 41-ton boat was believed to have been detained after entering the North’s exclusive economic zone, where foreign fishing boats are banned, the coast guard said in a statement.

There was no immediate confirmation from North Korea that its forces had impounded the squidding vessel. But the North Korean government, angry over an ambitious South Korean naval exercise due to end Monday, had said it would respond with “strong physical retaliation” and had warned civilian vessels to stay clear of the maritime border between the two Koreas.

Four South Koreans and three Chinese crew members were aboard the squidding vessel, the Daeseung 55.