Members of a South Korean navy salvage unit wait for soldiers searching for possible survivors from the sunken ship Cheonan. (Jo Yong-Hak/Reuters)

North Korea may have deliberately directed an underwater mine toward the South Korean naval ship that exploded and sank three days ago near a disputed maritime border, South Korea's defence minister told lawmakers Monday.

Defence Minister Kim Tae-young said military authorities have not ruled out North Korean involvement in the sinking of the Cheonan, which split apart within minutes after an explosion in the rear hull late Friday night.

Fifty-eight crew members were rescued from the Yellow Sea waters near Baengnyeong Island west of Seoul, but 46 others are believed trapped inside a rear segment of the ship with almost no chance of survival.

Divers rapping on the stern with hammers got no response Monday, military officials said.

Relatives of missing sailors complain to a military officer during a briefing on the rescue operation Monday at a naval base in Pyeongtaek, 70 kilometres south of Seoul. (Jung Yeon-je/Associated Press)

South Korean officials have been careful to say the exact cause of the explosion remains unknown, and that the rescue mission is still the top priority.

However, Kim told lawmakers in a frank update Monday in Seoul that North Korean involvement was one possibility.

"North Korea may have intentionally floated underwater mines to inflict damage on us," he said.

A mine placed by North Korea during the 1950-53 Korean War may also have struck the ship, he said.

North Korea planted about 3,000 Soviet-made mines off both coasts during the war, he said. There are no South Korean mines off the west coast, he said.

Officials have also said an internal malfunction may also be to blame.

The two Koreas remain in a state of war because their three-year conflict ended in a truce, not a peace treaty, in 1953. North Korea disputes the maritime border drawn by the United Nations in 1953, and the western waters — not far from where the ship went down — have been the site of three bloody skirmishes between North and South.

U.S. and South Korea military officials said there was not outward indication North Korea was involved in the sinking of the Cheonan.

North Korea's state media have made no mention of the ship. The North Korean military's first comments since the ship went down warned the U.S. and South Korea on Monday against engaging in "psychological warfare" by letting journalists into the Demilitarized Zone.

Still, the North Korean military was keeping a close watch on the search operation, the Joint Chiefs of Staffs said in a defence committee report cited by the Yonhap news agency.