Image caption Some 50,000 North Koreans and 500 Southerners work at Kaesong

North Korea has announced it is withdrawing all its workers from the joint-Korean Kaesong industrial zone and suspending operations there.

The move follows weeks of warlike rhetoric from Pyongyang after it was sanctioned by the UN for carrying out its third nuclear test in February.

Kaesong was established almost a decade ago and had been a symbol of co-operation between the two Koreas.

But a North Korean official said it could now be closed permanently.

In a statement, South Korea's Unification Ministry said the decision "cannot be justified in any way and North Korea will be held responsible for all the consequences".

The complex, just over the border in the North, employs more than 50,000 North Korean workers but is funded and managed by South Korean firms.

Pyongyang has already banned South Koreans from entering, but during a visit to the site, Kim Yang-gon, secretary of the party's Central Committee, said the North would now "temporarily suspend the operations in the zone and examine the issue of whether it will allow its existence or close it".

Analysis This is not the first time North Korea has threatened the future of the Kaesong industrial zone. But it is probably the most disruptive action taken in the factory complex's eight-year long existence. North Korea now says the survival of the complex depends on the attitudes of the South, but cynics will note that the announcement makes it clear that, for now, the withdrawal of the 53,000 workers is temporary. In the past - throughout the similar periodic crises - the North has, observers say, often taken actions to deliberately increase tension and then subsequently agreed to reduce it by undoing those actions for a price, economic or diplomatic. Given that Kaesong brings tens of millions of dollars of hard currency to the cash strapped country, there may well yet be a reprieve somewhere down the line for this rather battered symbol of inter-Korean cooperation. In pictures: Korea's Kaesong complex

The North's KCNA news agency quoted him as saying that South Korea and the US "insult the country's dignity and make the zone a starting point of war".

"How the situation will develop in the days ahead will entirely depend on the attitude of the South Korean authorities," Mr Kim said.

The statement made no reference to the nearly 500 South Koreans who are in Kaesong as managers.

One South Korean told the Associated Press he had heard nothing about the order from the North Korean government.

"North Korean workers left work at six o'clock today as they usually do. We'll know tomorrow whether they will come to work,'' he said.

Putin warning

Earlier, South Korean officials played down reports that the North could be about to carry out a nuclear test.

A defence ministry spokesman said the widely reported activity detected at the Punggye-ri underground test site appeared to be routine and that there was "no indication that a nuclear test is imminent".

UN Secretary General Ban Ki-moon said such a nuclear test would be a "provocative measure", and warned that North Korea cannot continue "confronting and challenging the authority of the Security Council and directly challenging the whole international community".

Russia and China have called for calm and a return to dialogue.

Timeline: Korean tensions 12 Dec: North launches a rocket, claiming to have put a satellite into orbit

North launches a rocket, claiming to have put a satellite into orbit 12 Feb: North conducts underground nuclear test

North conducts underground nuclear test 11 Mar : US-South Korea annual military drills begin

: US-South Korea annual military drills begin 30 Mar: North says it is entering a "state of war" with South

North says it is entering a "state of war" with South 2 Apr: North says it is restarting Yongbyon reactor

North says it is restarting Yongbyon reactor 3 Apr: North blocks South workers from Kaesong industrial zone

North blocks South workers from Kaesong industrial zone 4 Apr: South deploys warships to attack possible missiles from North

South deploys warships to attack possible missiles from North 5 Apr: North says it cannot guarantee safety of foreign embassies

North says it cannot guarantee safety of foreign embassies 8 Apr: South says North could be preparing another nuclear test Analysis: The North Korea threat Missile defences in the region Picking apart the bluster North Korea's missile programme

Speaking during a visit to Germany, Russian President Vladimir Putin said that as a neighbour of North Korea his country was "worried about the escalation" of tensions.

He warned there was a risk of a conflict on the Korean peninsula which would make the Chernobyl nuclear disaster "seem like a child's fairy tale".

The United Nations imposed tough sanctions on North Korea last month following its third nuclear test.

Pyongyang has responded by issuing almost daily threats to use nuclear weapons and saying it would restart its nuclear reactor.

The North has also shut down an emergency military hotline between Seoul and Pyongyang.

Last week it warned it would not be able to guarantee the safety of foreign embassy staff after 10 April, and that countries should begin evacuating their diplomatic staff.

The BBC's John Sudworth in Seoul says North Korea's state media have been broadcasting a continuing diet of war and retribution with programmes about biochemical war, nuclear war and military preparations dominating the listing.

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, he adds.

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.