In a sign of badly deteriorating relations across the world's last Cold War border, North Korea today kicked out eleven South Korean officials from Kaesong, a joint industrial centre created to bring the two sides together.

The surprise expulsions — executed at dawn and preceded with little of the usual verbal sabre-rattling from Pyongyang — marks the first serious cross-border headache for the new South Korean President, Lee Myung Bak.

The move was the North's most aggressive since it test-detonated an atomic device, and is a highly symbolic blow to the “sunshine policy” aimed at cooling tensions across the world's most heavily armed border.

Political analysts said that it was a gambit fully in line with North Korea's need to test the waters with the South's new leader. Unlike his more conciliatory predecessor, the nationalist Mr Lee has used his first month in power to present a sterner face to the North's unpredictable and nuclear-armed dictator, Kim Jong Il.

The new President has made it clear that the vast quantities of food and other aid — which, under Roh Moo Hyun, flowed north across the border with few strings attached — will now be linked to Pyongyang's behaviour. If Mr Kim continues to dither and obstruct on the North's nuclear programme, runs the new policy in Seoul, the billions of dollars of aid will stop.

Pyongyang, meanwhile, has been looking for an opportunity to embarrass the South's new President since his inauguration less than a month ago. The official media in Kim's “hermit kingdom” have not yet informed the North Korean people that their wealthier neighbours even have a new leader, let alone that he appears far tougher than Mr Roh.

The abrupt souring of relations is in very sharp contrast with Mr Roh's last serious act in office — a visit to Pyongyang last year and grand declarations of increased economic co-operation.

Long-term North Korea watchers were quick to dismiss Mr Roh's visit as worthless, and expected the terms of the joint declaration to unravel quickly, but the focus on the Kaesong Industrial Complex exploits especially strong symbolism.

The park, which is some 50 miles northwest of Seoul, employs 23,000 North Korean in about 70 South Korean-owned factories.

As part of its attempt to force Pyongyang to observe human rights and make progress on nuclear disarmament, Mr Lee's Cabinet has recently made it clear that Kaesong will not be expanded until the North steps into line.

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