SEOUL, ROK - A naval mine dispatched from the Democratic People's Republic of Korea (DPRK) may have struck the warship of the Republic of Korea (ROK) that exploded and sank near the Koreas' disputed sea border, the defense minister told lawmakers Monday, laying out several scenarios for the maritime disaster.

Relatives of the missing sailors of the sunken ROK's naval ship Cheonan, complain to a military officer during a briefing on the rescue operation at a naval base in Pyeongtaek, 70 km (43 miles) south of Seoul, March 29, 2010. [Agencies]

Defense Minister Kim Tae-young said there was no sign of a direct attack from thr DPRK, but military authorities have not ruled out DPRK involvement in the sinking of the Cheonan late Friday night.

The Joint Chiefs of Staff said the exact cause would remain unclear until the ship is salvaged after the rescue operation is over. On Monday, mothers beat their chests with grief as divers went down to search for survivors, returning to report hearing no sounds inside the underwater wreckage.

Kim, grilled by lawmakers on what happened Friday night, said the ship may have struck a mine left over from the war or deliberately dispatched from DPRK.

He insisted there were no ROK's mines off the west coast, and ruled out a torpedo attack from DPRK, which would have been spotted by radar.

Officials have also said an internal malfunction may be to blame. The 1,200-ton Cheonan is designed to carry weapons, and was involved in a previous skirmish with DPRK.

US and ROK's military officials said there was no outward indication that DPRK was involved in the sinking of the Cheonan.

However, "neither the government nor the defense ministry has ever said there was no possibility of DPRK's involvement," Kim said.

The DPRK military was keeping a close watch on the search operation, the Joint Chiefs of Staffs said in a defense committee report cited by the Yonhap news agency.

But Pyongyang's state media have made no mention of the ship.

ROK's President Lee Myung-bak urged rescuers not to give up hope.

But the prospect of pulling anyone out alive seemed dim Monday. Any navy crewmen who managed to seal themselves inside their watertight cabins would have run out of air by Monday night since the supply of oxygen in the cabins was estimated to last up to 69 hours, military officials said.

Rough waves prevented divers from gaining access to the wreckage. Divers finally reached the ship's rear segment, where most of the missing were believed trapped, on Monday. They hammered on the hull but got no response, Rear Adm. Lee Ki-sik of the Joint Chiefs of Staff told reporters.

The US Navy sent four ships and a team of divers to join the search, said Lt. Anthony Falvo, a spokesman for the 7th Fleet in Japan.

Anguished relatives waited for news at a naval base in Pyeongtaek, south of Seoul, some pounding their chests with grief, others shouting their displeasure at the slow pace of the rescue.

"My baby, my baby," one mother murmured, briefly losing consciousness.

"The navy is rotten to the core!" a man yelled, lunging at soldiers standing arm in arm to block angry family members from barging into the command center.