South Korean troops fired dozens of warning shots at North Korean soldiers who crossed the heavily defended border, ratcheting up tensions after Pyongyang said it would test a nuclear weapon.

Military authorities here said the troops opened fire after five North Korean soldiers crossed into the no-man’s land dividing the two sides which remain technically at war after the 1950-53 Korean War ended with an armistice.

“The North Korean soldiers crossed the military demarcation line and came some 30 meters (yards) south. They retreated after our troops fired warning shots,” an official at the Joint Chiefs of Staff told AFP.

“We fired some 60 warning shots, including live ammunition, with intervals,” he said. The North Koreans, one of who was armed with a rifle, returned to their side about an hour later.

It marked the first time since May that North Korean soldiers have crossed the highly sensitive border and comes amid heightened tensions following Pyongyang’s announcement that it plans to test a nuclear weapon.

The UN Security Council unanimously agreed Friday to press

North Korea to drop its plans to test an atom bomb, which Japan said could be detonated this weekend.

Pyongyang gave no immediate response to the UN move but renewed its call for the United States to withdraw its forces from

South Korea and said the risk of war was increasing.

“The ongoing ‘reorganisation’ of the US forces in South Korea is part of the arms buildup and a prelude to a war of aggression against the DPRK,” the North Korean news agency KCNA said, quoting from a statement by the Committee for the Peaceful Reunification of the Fatherland.

The United States has stationed tens of thousands of troops here since the end of the Korean War but wants to cut the current 29,500 to 25,000 by 2008. It also wants flexibility to deploy them elsewhere in the region if necessary.

“Due to such moves the situation is getting tenser and the danger of a nuclear war is further increasing on the Korean Peninsula as the days go by,” the North Korean statement said.

“The US should pull its aggression forces out of South Korea and its vicinity at once.”

Meanwhile, neighbouring Japan and South Korea on Saturday welcomed the

United Nations’ tough line and warned the reclusive North to be ready for the consequences if it follows through with plans to test a nuclear weapon.

“Should North Korea carry out a nuclear test despite the common concern expressed by the international community, Japan believes the Security Council should swiftly adopt a resolution that contains stern measures,” Japanese Foreign Minister Taro Aso said in a statement.

South Korea also warned the North would be held “wholly responsible for all the consequences” if it exploded an atom bomb and urged it to return to six-nation disarmament talks.

In addition, a wide-ranging diplomatic dialogue over the nuclear crisis is set to get underway among North Korea’s neighbours.

Japanese Prime Minister Shinzo Abe was due to visit China on Sunday and South Korea on Monday in his overseas meetings since taking office late last month. The premier said he would seek common ground with his neighbours over the nuclear crisis.

Japan took a hardline stance on North Korea, including economic sanctions, when the communist state test-fired missiles in July, but South Korea called for a calm response and accused Tokyo of heightening tensions.

China, North Korea’s main political ally, has also favored a more conciliatory approach towards Pyongyang.

In Seoul, a senior foreign ministry official said Saturday that Chun Yung-Woo, South Korea’s chief delegate to the six-party talks, would visit Beijing on Monday for talks with his Chinese counterpart Wu Dawei.

“Aside from messages of warning to North Korea, diplomatic efforts to resolve this issue through dialogue and negotiations are being pursued as well and consultations with the Chinese side are being carried out in this context,” the official told Yonhap news agency.

He said North Korea was under round-the-clock surveillance and that so far the South Koreans had spotted no unusual movements that might suggest a nuclear test was imminent.

But Japanese vice foreign minister Shotaro Yachi, on a visit to Washington, said Pyongyang’s resolve to test its first atom bomb should not be underestimated.

“We discussed the possibility that the test would occur this weekend,” Yachi said after talks with US deputy national security adviser Jack Crouch, in footage broadcast by Japanese public broadcaster NHK.

“They will probably go ahead and do it as they had that tone in their declaration,” Yachi said. “It possibly means they are already very prepared.”