North Korea preparing nuclear test, South Korea says

North Korea may be preparing for a fourth nuclear test, South Korean officials say.

South Korean Unification Minister Ryoo Kihl-jae told lawmakers there were signs of increased activity in the North's main nuclear test site.

North Korea has stepped up its habitual fiery rhetoric against the US and South Korea in recent weeks.

Correspondents say it is unclear whether a test is being prepared or it is a ruse to boost the sense of crisis.

Mr Ryoo did not elaborate on the specific intelligence which led South Korea to suspect the North was set to carry out its fourth nuclear test.

But when asked about a news report that North Korea had stepped up activity at the underground site it has used in previous tests, he said "there are such signs".

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Timeline: Korean tensions

• 12 Dec: North launches a rocket, claiming to have put a satellite into orbit
• 12 Feb: North conducts underground nuclear test
• 11 Mar: US-South Korea annual military drills begin
• 30 Mar: North says it is entering a "state of war" with South
• 2 Apr: North says it is restarting Yongbyon reactor
• 3 Apr: North blocks South workers from Kaesong industrial zone
• 4 Apr: South deploys warships to attack possible missiles from North
• 5 Apr: North says it cannot guarantee safety of foreign embassies
• 8 Apr: South says North could be preparing another nuclear test

• Missile defences in the region
• How potent are North's threats?
• North Korea's missile programme

Kim Min-seok, a spokesman for South Korea's defence ministry, said it was possible that the North could fire a ballistic missile and conduct a nuclear test at the same time.

But the official Yonhap news agency quoted him as playing down the threat of an imminent test, saying there were several facilities at the nuclear test site so the movement of vehicles and people there was expected.

"Currently, there is no new movement to add on to the previous briefing," he said.

Newspaper JoongAng Ilbo quoted an unnamed South Korean official as saying that South Korean intelligence had detected "increased activity of labour forces and vehicles" at the Punggye-ri nuclear test site in the country's north-east.

"We are closely monitoring the ongoing situation, which is very similar to the situation ahead of the third nuclear test," the official told the newspaper.

"We are trying to figure out whether it is a genuine preparation for a nuclear test or just a ploy to heap more pressure on us and the US."

Embassy warning

The United Nations imposed tough sanctions on North Korea last month following its third nuclear test. Pyongyang responded by stepping up angry rhetoric, including threats to use nuclear weapons and restarting its nuclear reactor.

In recent weeks, the North has shut down an emergency military hotline between Seoul and Pyongyang, stopped South Koreans from working at the Kaesong joint industrial complex and warned it would not be able to guarantee the safety of foreign embassy staff in the event of a war.

On Monday, the North's state news agency said a senior official had visited the Kaesong complex and told workers there to prepare for all possible developments.

Tension has heightened in the Korean peninsula

But some analysts have suggested that the rhetoric is in large part designed to shore up the standing of a young, inexperienced leader, Kim Jong-un, in the eyes of his own people, says the BBC's John Sudworth in Seoul.

North Korea's state media have been broadcasting a continuing diet of war and retribution, our correspondent adds, with programmes about biochemical war, nuclear war and military preparations dominating the listing.

Meanwhile, Japan's defence ministry said the country's armed forces have been ordered to shoot down any North Korean missile headed towards its territory.

Over the weekend, the US cancelled a scheduled test of its Minuteman 3 ballistic missile, citing concerns that it could be misinterpreted by Pyongyang.