SEOUL — North Korea's military threatened Friday to strike back with deadly firepower if South Korea goes ahead with a live-fire drill on a border island which the communist state shelled last month.

The North "will deal the second and third unpredictable self-defensive blow" to protect its territorial waters if the South holds the one-day drill scheduled sometime between Saturday and Tuesday, it said.

"It will be deadlier than what was made on November 23 in terms of the powerfulness and sphere of the strike," said the military statement carried on the North's official news agency.

The bombardment of Yeonpyeong island last month killed two marines and two civilians, injured 18 people and damaged dozens of homes, and came after a firing drill into the sea by South Korean marines based on the island.

The South, outraged at the first shelling of civilian areas since the 1950-53 war, has fortified Yeonpyeong with more troops and artillery and vowed to use air power against any future attack.

Its military has said artillery will be aimed away from the North as usual during the upcoming drill, but it will respond strongly if provoked.

Members of the US-led United Nations Command are scheduled to observe the drill and about 20 US soldiers will play a supporting role.

But a top US general Thursday voiced concern over a possible "chain reaction".

General James Cartwright, vice chairman of the US Joint Chiefs of Staff, said the drill was being held on a "well-established and well-used" range in a transparent way, but could draw a North Korean reaction.

"What we worry about obviously is... if North Korea were to react to that in a negative way and fire back at those firing positions on the islands, that would start potentially a chain reaction," Cartwright told reporters.

"What you don't want to have happen out of that is for... us to lose control of the escalation."

Seoul military spokesmen had no immediate comment on whether the drill would go ahead. The defence ministry, in a statement, suggested it would be held.

"Our military's stance is that we do not need to react to every single threat and unreasonable statement," it said.

US State Department spokesman Philip Crowley said Thursday there was "nothing provocative or unusual or threatening about these exercises".

The North Koreans "would be very unwise to react to what South Korea has announced", Crowley said.

Yeonpyeong is just south of the Yellow Sea border drawn by United Nations forces after the war, which the North refuses to recognise. It claims the seas around the island as its own maritime territory.

In an earlier message, Pyongyang's official website Uriminzokkiri warned that another war with South Korea would involve nuclear weapons.

"Because of the South Koreans' reckless war policies, it is not about war or peace on the Korean peninsula, but when the war will break out," the website said.

"If war breaks out, it will lead to nuclear warfare and not be limited to the Korean peninsula."

The North frequently claims nuclear war is imminent. But conventional forces are on high alert after last month's shelling.

Pyongyang's disclosure last month of an apparently working uranium enrichment plant -- a potential new source of bomb-making material -- also heightened regional security fears.

US politician Bill Richardson, a veteran troubleshooter with North Korea, is paying a private visit to Pyongyang to try to ease tensions.

And the US envoy to stalled six-party talks on the North's nuclear disarmament, Sung Kim, held talks in Seoul Friday with his South Korean counterpart Wi Sung-Lac.

In Beijing, a US delegation led by Deputy Secretary of State James Steinberg wrapped up three days of "useful" discussions on the Korean peninsula situation, the US embassy said.

The United States, China, the two Koreas, Japan and Russia are members of the denuclearisation forum which the North abandoned in April 2009, a month before its second atomic weapons test.

Host China along with Russia is trying to revive the forum to ease the crisis, and the North says it is willing to talk. But the United States, South Korea and Japan say it must first mend ties with the South and show genuine seriousness about abandoning its nuclear drive.

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