By Kim Yoo-chul

North Korea has seized a South Korean fishing boat that was apparently operating in the East Sea near the North's territorial waters, the Ministry of Unification said Sunday.

The 41-ton boat, "Daeseung 55," was crewed by four South Koreans and three Chinese when it was seized.

"It appears that the North Korean authorities are questioning the seven crew members," a ministry spokesman said.

The vessel was towed by an unnamed North Korean patrol boat near the North's exclusive economic zone in the East Sea, late Sunday afternoon, according to South Korean Coastguard reports.

The captain of the boat reported "normal conditions" around 6:30 p.m. Saturday when contacted by South Korean maritime authorities via regular radio contact.

"But we lost the signal around 5:30 a.m. Sunday and later confirmed via satellite that the boat was being towed by a North Korean patrol boat heading to Pyongyang's northeastern port of Songjin," a coastguard official said.

The ship, which sailed from the port of Pohang, North Gyeongsang Province, on August 1 was expected to return around September 10, the official said.

It's not yet clear whether the fishing boat was engaged in illegal fishing.

The Coast Guard said it was investigating whether the boat trespassed into the North's exclusive economic zone and called for the prompt release of the crew in line with international law and customs.

The incident came amid deadlocked inter-Korean relations.

South Korea and the U.S. recently held massive naval drills south of the East Sea border between the Koreas, after they accused Pyongyang of sinking a South Korean warship in the West Sea in March, killing 46 sailors.

South Korea is mobilizing some 4,500 sailors and airmen for its own drills that have been under way off the west coast of the Korean Peninsula since Thursday.

The North, which denies its role in the sinking of the frigate Cheonan, has threatened "strong physical retaliation" for the exercise.

The unification ministry was cautious over the possible impact of the incident on the soured ties between the two Koreas.

"Let's see what happens," the ministry spokesman said.