North Korean authorities seized a South Korean fishing boat and its crew today off the divided peninsula's eastern coast, the South's coastguard said. The incident comes amid heightened tensions over the sinking of a South Korean navy ship earlier this year.

Four South Korean and three Chinese fishermen were being questioned for an alleged violation of the North's exclusive economic zone, the coastguard said. The boat was being taken toward the North Korea's eastern port of Songjin, it added.

One of the South Korean fisherman said via a satellite phone that his boat was being towed by a North Korean patrol, according to the coastguard.

It is not clear where exactly the 41-tonne boat was operating when it was seized. The vessel left South Korea's south-eastern port of Pohang on 1 August and was due to return home on 10 September.

South Korea called on the North to return the boat and its crew. However, such a prospect is potentially complicated by tension over the March sinking of a South Korean warship off the western coast, which Seoul blamed on North Korea. South Korea has also conducted naval drills off the western coast, including areas near the two countries' disputed sea border. The exercises, which end tomorrow, were aimed at strengthening South Korea's ability to counter any North Korean provocations.

North Korea – which has denied involvement in the sinking – warned last week it would "counter the reckless naval firing projected by the group of traitors with strong physical retaliation" and advised civilian ships to stay away from the maritime border.

Maritime incidents involving fishing boats and other commercial vessels occur from time to time between the two Koreas. While most are resolved amicably, the rival navies engaged in three deadly skirmishes near their disputed western sea border in 1999, 2002 and November last year.

Last August, North Korea freed four South Korean fishermen after detaining them for a month for illegally entering North Korean waters.