North Korea said Monday it would bolster its nuclear weaponry with an unspecified new method in response to what it called US hostility and recent developments.

"The recent disturbing development on the Korean peninsula underscores the need for the DPRK (North Korea) to bolster its nuclear deterrent in a newly developed way to cope with the US persistent hostile policy toward the DPRK and military threat toward it," a foreign ministry spokesman said.

The spokesman, quoted by the official news agency, did not elaborate.

North Korea announced on May 12 it had conducted a nuclear fusion reaction, a process that can be used in making a hydrogen bomb. It did not link the allegedly successful experiment to its atomic weapons programme.

South Korean officials said they had detected an abnormally high level of xenon gas -- a by-product of atomic tests -- on May 14, but added there was no evidence a test had been carried out.

Last September the communist state announced it had reached the final stage of enriching uranium, a second way of making nuclear bombs in addition to the North's original plutonium-based operation.

The North has reacted angrily to South Korea's moves to censure it at the United Nations for the sinking of one of Seoul's warships in March. It has denied responsibility and threatened a military response to any UN action.

Monday's statement followed the disclosure last week that the United States studied a plan for tactical nuclear strikes on North Korea in 1969, as one possible option in response to the downing of a US spy aircraft.

The disclosure showed that the US "has always watched for a chance to use nuclear weapons" against the North, the ministry spokesman said.

"Historical facts prove that the DPRK was quite right when it made a decision to react to nukes with a nuclear deterrent."

The hardline communist state has carried out two atomic weapons tests, in 2006 and 2009. It is thought to have enough plutonium to make around six nuclear weapons, but it is unclear whether it has the means of delivering them.

In a separate statement, the North's military accused the United States Monday of bringing what it called heavy weapons into the border truce village of Panmunjom.

It warned of "strong military countermeasures" at the village, a top tourist attraction for visitors from the South, unless they are withdrawn.

A multinational investigation concluded in May that a North Korean submarine torpedoed the South Korean warship Cheonan in March with the loss of 46 lives.

Tensions rose sharply after the South, with strong US support, announced non-military reprisals and sought UN Security Council censure.

Permanent veto-wielding council members China and Russia have been hesitant about publicly blaming the North.

US President Barack Obama, speaking on the sidelines of a G20 summit in Toronto Sunday, said he held talks with Chinese President Hu Jintao and had been "blunt" with him on the issue of North Korea.

While he understood that North Korea and China were neighbours, Obama said: "I think there's a difference between restraint and willful blindness to consistent problems.

"My hope is that President Hu will recognise as well that this is an example of Pyongyang going over the line."

In a communique after a Group of Eight meeting on Saturday, members including Russia noted the investigation results, condemned "in this context" the warship attack and demanded that the North refrain from hostile actions against the South.

South Korean media noted, however, that the statement did not explicitly accuse the North of sinking the ship.

North Korea said Sunday it was open to inter-Korean military talks to address the sinking, but it repeated refusals to deal with US "imperialist aggressor" forces on the issue.

It repeated demands to send its investigators to the South to examine the evidence on the sinking, a condition which Seoul has refused.