SEOUL • The US envoy to the United Nations made a trip to the world's most fortified border separating the two Koreas yesterday and vowed to further isolate the North over its nuclear weapons programme.

Ms Samantha Power's trip to the heavily guarded Demilitarised Zone came amid growing concerns that Pyongyang may carry out another nuclear test or launch a long-range rocket today to mark the anniversary of the founding of its ruling Workers' Party and the 10th anniversary of its first nuclear test on Oct 9, 2006. Its fifth and most powerful nuclear test was carried out on Foundation Day on Sept 9.

A US-based monitoring group, 38 North, has said satellite imagery showed an increase in activity at the North's nuclear test site that could signal preparations for a new test.

Ms Power, who arrived in Seoul last Saturday, told a press conference that the US will use its military as a deterrent to the North's threat.

She said: "While Security Council resolutions are one tool in our toolbox... we are committed to using all the tools in our toolkit to address this serious threat including the diplomatic pressure that we are mobilising around the world to convince other nations to isolate the regime."

Her visit to the region, which included a stop in Tokyo last week, comes amid a push for tougher Security Council sanctions after the North's fifth nuclear test last month, in defiance of a series of UN resolutions.

The US and South Korea have been pushing governments around the world to take tougher actions to pressure the North, including expanding a trade ban on coal, fuel and other resources.

Ms Power also visited a settlement support centre for North Korean defectors yesterday, reported the Yonhap news agency.

Government insiders told Yonhap the trip to the Hanawon centre in Anseong, about 77km south of Seoul, is part of Washington's drive to make an issue of deplorable human rights abuses in North Korea.

On arrival last Saturday the US ambassador said she wanted to directly hear the plight of those who have fled the North and, based on what she learnt, go back to New York to negotiate fresh sanctions with other countries.

She had said it is deplorable that the North Korean regime threatens and abuses its people.

She added: "We also seek to shine the world spotlight on the worst crimes with really no parallel in this world."

Yesterday marked 10 years to the day that North Korea carried out its first nuclear test on Oct 9, 2006 - an underground detonation with such a low yield that it was widely seen as a failure.

But the North's weapons programme has progressed by leaps and bounds since then - despite rounds of increasingly tough international sanctions - and has notably accelerated under current leader Kim Jong Un.

When North Korea first claimed to have launched a ballistic missile from a submarine in May, there was much derision because it appeared that the photographs had been doctored. Fast forward to August, and North Korea carried out a successful test from a submarine.

Associate Professor Vipin Narang, a political scientist and expert on nuclear proliferation at the Massachusetts Institute of Technology, said: "It seems like North Korea is trying to build a wide array of delivery platforms so that they're able to hit Japan, South Korea, American assets in Asia, and eventually, the homeland."

He added that North Korea no longer appears to be following the Cold War model of having nuclear weapons for mutually assured destruction, but seems to be looking more like Pakistan, which has adopted a strategy of "asymmetric escalation" - being able to use a nuclear arsenal against a conventional attack. "This programme is no longer a joke."

REUTERS, KOREA HERALD/ASIA NEWS NETWORK, WASHINGTON POST