S Korea, US troops raise alert
Seoul - South Korean and US troops went on higher alert on Thursday after North Korea threatened an attack on the South and Washington stressed its determination to defend its ally.
Seoul's defence ministry said air and ground forces were keeping a closer watch on the tense land and sea border after the communist North announced it is abandoning the armistice in force since the end of the Korean War in 1953.
Tensions have risen sharply since the North on Monday tested a nuclear weapon several times more powerful than the one it detonated in 2006. It followed up by test-firing five short-range missiles.
"We are maintaining a tight defence posture to prevent the North's military provocations," said ministry spokesperson Won Tae-Jae. "The military will deal sternly with provocative acts."
The United States on Wednesday stressed its resolve to defend Japan and South Korea amid what it called North Korean "sabre-rattling and bluster".
A Western diplomat at the United Nations said leading powers are committed to tougher sanctions against Pyongyang.
The US headed a UN command which fought for the South in the Korean war. It still stations 28,500 troops there.
"I want to underscore the commitment the United States has, and intends always to honour, for the defence of South Korea and Japan," said US Secretary of State Hillary Clinton.
A declaration of war
Pyongyang's latest display of anger was prompted by Seoul's decision to join the US-led Proliferation Security Initiative (PSI) to stop the spread of weapons of mass destruction.
This can involve stopping and searching ships, although the South has said North Korean merchant ships can still cross its sea borders under a 2005 accord.
The North, however, described Seoul's decision as akin to a declaration of war.
It said its military would no longer be bound by the armistice.
"Any tiny hostile acts against our republic, including the stopping and searching of our peaceful vessels... will face an immediate and strong military strike in response," the military said.
It was the fifth time in 15 years that the North has threatened to tear up the armistice, according to the White House.
"I think their actions would be better focused on living up to their rights and obligations," said spokesperson Robert Gibbs.
"We're certainly concerned and take any threat seriously," Gibbs said. "But my sense is they're trying to get renewed attention through sabre-rattling and bluster and threats."
UN working on resolution
Clinton said "a unified international community, including China and Russia" was crafting a UN response to punish North Korea for Monday's blast, "rein in" the regime and resume efforts to dismantle its nuclear arsenal.
The Security Council has condemned Monday's test and is working on a resolution.
The Western diplomat said there was a clear commitment by the five permanent members plus Japan and South Korea to seek new sanctions, although discussions were still in the early stages.
He said proposals include extending the list of entities targeted for travel bans or financial sanctions, a broader arms embargo, tougher inspections of cargo, a freeze on North Korean assets abroad and denial of access to international banking and financial services.
The full council was expected to meet early next week to consider a draft resolution, he said.
A years-long diplomatic effort to shut down the North's nuclear programme appeared to be paying off back in 2007, when Pyongyang shut down the plants at Yongbyon which produced weapons-grade plutonium.
But the North's policy has become noticeably harder-line since last summer when leader Kim Jong-Il, now 67, reportedly suffered a stroke.
Several analysts believe Kim staged the test to shore up his authority as he puts his succession plans in place.
After the Security Council censured its April 5 rocket launch and tightened existing sanctions, the North announced it was quitting six-nation nuclear disarmament talks and would restart Yongbyon.
It also made good on Monday on its threat of a second nuclear test, exploding a device said by analysts to be up to four or five times more powerful than in 2006.
However a US defence official, contradicting South Korean media reports, said there were no signs the North had restarted work to make more weapons-grade plutonium.
Commercial satellite photos released on Wednesday showed no steam coming from the plant, according to the Institute for Science and International Security.