A stray South Korean mine could not have sunk one of the country's warships, the defence ministry said Wednesday, in comments likely to heighten suspicions that North Korea was to blame.

"Its technically not possible," said spokesman Won Tae-Jae, dismissing suggestions that a South Korean mine planted in the Yellow Sea in the 1970s might have blown the corvette apart.

The mysterious sinking of the 1,200-tonne Cheonan near the tense inter-Korean border with the loss of 46 lives has spawned a series of theories -- including a stray mine, a grounding, an onboard explosion or even metal fatigue.

Investigators who examined the salvaged ship sections now say an external explosion hit it on March 26. Defence Minister Kim Tae-Young said Sunday a heavy torpedo attack was one of the likeliest causes.

The South laid around 100 mines near the Yellow Sea border in the 1970s before retrieving them in 2008.

Won said that even if one of the mines had been overlooked it could not have exploded now because additional wiring is needed to make them operational.

The South has not accused the North of an attack, pending the outcome of a multinational probe, but is weighing its response in case hard evidence emerges.

Further souring ties, North Korea Wednesday barred access to more South Korean-owned properties at a resort in the North in protest at the Seoul government's ban on tours there.

Last week Pyongyang confiscated five Seoul government-run properties at the Mount Kumgang resort and this week began to "freeze" privately owned premises.

Unification ministry spokeswoman Lee Jong-Joo said assets frozen Wednesday included a restaurant, a beachside hotel and parts of an 18-hole golf course.

She said the move violated inter-Korean agreements and international trade norms, and Seoul would monitor them closely before announcing its response.

Seoul suspended tours in July 2008 after a North Korean soldier shot dead a South Korean housewife who strayed into a military zone.

The sanctions-hit North, which once earned millions of dollars a month from the business, is pressuring the South to lift its ban.

An unidentified official told Yonhap news agency that Seoul has not ruled out a military response if Pyongyang's involvement in the ship sinking is proved, but would likely take the case to the United Nations Security Council.

South Korea will brief permanent Council members China and Russia on the results of its investigation, the official said.

South Korean and US officials have indicated that six-nation nuclear disarmament talks cannot resume while the sinking remains a mystery.

The forum groups the two Koreas, host China, Russia, Japan and the United States. The North walked out a year ago but has expressed conditional interest in coming back.

"From now on the issue of resuming the six-party talks will be handled in connection with the Cheonan case and in this atmosphere, it is difficult to resume the talks," the official said.

"It will take several months before the talks resume."

JoongAng Ilbo newspaper, quoting a source, said China has agreed with South Korea and the United States to wait for the Cheonan issue to settle before resuming the nuclear dialogue.