North Korea is a 'growing threat' to world security warns US as stealth bombers patrol coast Employees at Kaesong factory complex barred from crossing the border Industrial zone was a sign of positive co-operation between neighbours Chinese soldiers and weaponry massing on border with Korean Peninsula John Kerry warns that North is 'provocative, dangerous and reckless' Yongbyon nuclear site set to be restarted after being closed for six months U.S. stealth bombers were today patrolling the border between North and South Korea as part of military exercises which have inflamed tensions in the region. Chinese troops have been placed on a heightened state of alert along the country's frontier with North Korea after a series of warlike statements and actions from the pariah state. The North today blocked South Korean workers from entering a jointly run factory zone which is one of the few signs of positive relations between the neighbouring countries. The move to bar South Koreans from going to work at the Kaesong industrial complex comes a day after North Korea announced that it would re-open a nuclear facility which has been closed for six years. Patrol: An F-22 stealth fighter jet takes off from a U.S. Air Force base in South Korea today Base: American stealth fighters at the Air Force facility in Osan, south of Seoul On the runway: Both U.S. and Chinese forces have been amassing near North Korea in recent days Lining up: South Korean soldiers man the tanks at a drill near the border with neighbouring North Korea U.S. Secretary of State John Kerry yesterday spoke out against North Korea's latest aggressive moves, calling them 'provocative, dangerous and reckless', while Defence Secretary Chuck Hagel said the country was a 'growing threat' to the world's security. South Korean officials said that workers were being allowed to return home from Kaesong, which is on the north side of the border. But around 480 people who had planned to travel to the park today were denied entry, with North Korea blaming the decision on recent political developments. The Kaesong industrial park started producing goods in 2004 and has been an unusual point of co-operation in an otherwise hostile relationship between the Koreas. Blocked: South Korean vehicles and workers have been banned from entering North Korea today Turning back: Vans which have been refused entry to the Kaesong industrial complex return to South Korea Locked out: A South Korean worker driving away from the border after being denied entry North and South Korea do not allow their citizens to travel to the other country without approval, but an exception had previously been made each day for the South Koreans working at Kaesong. About 120 South Korean firms run factories in the border town of Kaesong, with 53,000 North Koreans working there. Using North Korea's cheap labour, the Kaesong complex produced $470million worth of goods last year. In 2009, North Korea closed its border gate in anger over U.S.-South Korean military drills, leaving hundreds of South Korean workers stranded in Kaesong for several days. The park later resumed normal operations. 'I feel worried that I'm unable to do business and also feel anxious,' Joe In-suk, a South Korean who had planned to travel to Kaesong, said today at a border checkpoint. Checkpoint: A South Korean soldier checks credentials in the middle of the traffic jam near the border On guard: South Korean soldiers at the nearest railway station to the border Empty: The Kaesong factory will be unable to function without a steady stream of South Korean workers Security: Vehicles at the demilitarised zone near the Kaesong complex, with is on the north side of the border If North Korea continues to deny entrance to South Korean workers, the complex would have to shut down because Kaesong factories cannot operate production lines without supplies of raw materials sent regularly by truck from the South to the North. A South Korean manager whose company runs a factory in Kaesong was worried that buyers would drop future orders if North Korea continued to block workers and supplies from the South. 'For some companies, today's move must have already dealt them a blow,' the manager said. We cannot produce goods without raw material. Seoul's Unification Ministry urged Pyongyang to 'immediately normalise' cross-border traffic in and out of Kaesong. Preparing for war? A Marine from South Korea observes a line of K-55 howitzers during military drills Troops: South Korean Marines during military drills today as it was revealed Chinese soldiers were amassing near the country's border with the North Defence: The South has been conducting a series of drills aimed at its volatile neighbour Mr Kerry warned North Korea to stop provoking the South and the U.S. after weeks of threats from the secretive state and its leader Kim Jong Un. 'The bottom line, very simply, is that what Kim Jong Un has been choosing to do is provocative, it is dangerous, reckless, and the United States will not accept the DPRK as a nuclear state,' he said. He said that any move to re-start operations at the Yongbyon nuclear plant would be 'a direct violation' of North Korea's international commitments and a 'very serious step'. 'It would be a provocative act and completely contrary to the road we have travelled for all these years,' he said. Mr Hagel said that North Korea's nuclear threats were a 'growing threat' in a telephone call to his Chinese counterpart. Target practice: North Korea has released video footage of soldier firing at cartoon-like targets of U.S. soldiers China has warned troops stationed in its north-east that they should be ready for mobilisation in case conflict breaks out between the Koreas, according to the Washington Free Beacon. Unusual numbers of soldiers and armoured vehicles have been reported in the region, and commanders have apparently been ordered to be on the highest degree of readiness. U.S. officials said that China was believed to be responded to the warlike threats from its North Korean ally, adding that the country had conducted live-firing drills near the border. The top American commander in South Korea has described the situation as 'volatile' and 'dangerous', saying he fears a 'miscalculation' could lead to full-blown military conflict. General James Thurman told ABC News that Kim was trying to 'intimidate the South Koreans and intimidate the region', but said the Americans were 'calm' and 'confident' at the possibility of confrontation. Co-operation: South Korean foreign minister Yun Byung-se with U.S. Secretary of State John Kerry Threat: Kim Jong Un, pictured centre at a meeting of North Korea's parliament yesterday, has vowed to re-open the country's nuclear facilities December 2011: Kim Jong Il died and his son Kim Jong Un was named his successor. April 2012: To commemorate the 100th anniversary of the birth of the country's founder Kim Il Sung, Pyongyang launched a long-range rocket which fell into the sea. August 2012: Ahead of the annual US-South Korean drills, Kim Jong Un announced that the North Korean army was ready to deal 'deadly blows December 2012: North Korea successfully launched a rocket, boosting the credentials of its new leader and stepping up the threat the isolated and impoverished state poses to opponents. The rocket was labelled by the United States, South Korea and Japan as a test of technology that could one day deliver a nuclear warhead. January 2013: North Korea announced it was planning a new nuclear test, raising the confrontation with the US to a whole new level. February 12: North Korea conducted its third nuclear test and warned that further measures would follow if the US continued its "hostility" against the North. The UN subsequently imposed more sanctions on the county. March 29: Following a mock bombing of North Korea by a US B-2 stealth bomber during a joint military drills, Pyongyang ordered rocket units be put on standby to fire on US bases in the South Pacific. March 30: North Korea declared it was entering a 'state of war' against its Southern neighbour. North Korean officials said yesterday that at the Yongbyon facility a graphite-moderated five-megawatt reactor, which generates spent fuel rods laced with plutonium, would be restarted soon. The UN chief says he fears North Korea is on a collision course with other nations that could lead to war. Secretary-General Ban Ki-moon says the isolated Asian nation appears to be 'on a collision course with the international community' amid rising tensions on the Korean Peninsula. Ban, a former South Korean foreign minister, said today in Andorra 'the current crisis has already gone too far' because of escalating tensions raised by North Korea's threats of war almost daily against the United States and South Korea. He said international negotiations are urgently needed but he is 'convinced that no one intends to attack' North Korea. Pyongyang has sought disarmament-for-aid talks with Washington and more domestic loyalty by portraying North Korean leader Kim Jong Un as a powerful commander. 'The General Department of Atomic Energy... decided to adjust and alter the uses of the existing nuclear facilities, to begin with,' KCNA reported. Warning sign: North Korea's December 12 rocket launch shows the country has likely developed the technology to fire a missile that could reach the U.S. West Coast Stung by criticism that its response to the shelling of a South Korean island in 2010 was weak, Seoul has also threatened to target North Korean leader Kim Jong Un and to destroy statues of the ruling Kim dynasty in the event of any new attack, a plan that has outraged Pyongyang.