N Korea approves nuclear strike on US North Korea says it has approved nuclear attack on the United States in its latest threat as US Defence Secretary Chuck Hagel warned Pyongyang to back down. The White House and Pentagon would be notified that "reckless operations" using nuclear weapons had been approved, North Korea's supreme military command said in a statement quoted by South Korean news agency Yonhap. "The moment of explosion is approaching fast," the North Korean military said, warning that war could break out "today or tomorrow." Analysts have said North Korea is not technologically capable of carrying out a strike on the US mainland but could target US troops stationed in South Korea or Japan. The North Korean announcement came shortly after the US said it will deploy a ballistic missile defence system on its Pacific island of Guam as part of Washington's efforts to beef up its military presence in the region. The land-based Terminal High Altitude Area Defense System (THAAD) "will strengthen defence capabilities for American citizens in the US territory of Guam and US forces stationed there," the Pentagon said. Hagel called on Wednesday on Pyongyang to back down from its "dangerous rhetoric." We take those threats seriously. We have to take those threats seriously," Hagel said at the National Defence University in Washington. The US had taken measured, reasonable steps in response to North Korea's recent moves, he said, noting the danger of being wrong about the seriousness of the threats. "It only takes being wrong once, and I don't want to be the secretary of defence who was wrong once," Hagel said. North Korea on Wednesday blocked South Korean employees from entering an industrial complex operated jointly by the two countries, only allowing workers to leave, a Seoul official said. In recent days, Pyongyang has threatened to close the Kaesong facility, which lies at the western end of the border inside its territory, amid heightened tension on the peninsula. It notified Seoul that it would block entry to Kaesong for South Koreans, a spokesman for the South Korean Unification Ministry was quoted as saying by the Yonhap News Agency. Workers were waiting on the south side of the checkpoint, a ministry spokeswoman said. Workers would be allowed to leave normally, Pyongyang reportedly said. The Kaesong complex, 10km north of the border, opened in 2004 as part of reconciliation agreements between the once-warring neighbours. The park is one of isolated North Korea's only sources of hard currency, earning an estimated $US80 million ($A76.90 million) a year. South Korean Defence Minister Kim Kwan Jin told legislators from the conservative ruling Saenuri Party that using military force to free hundreds of workers at the industrial area was a possibility if they were held hostage, Yonhap said. Officials in Seoul said the likelihood of North Korea holding South Korean workers hostage was slim, according to the report. North Korea last week cut the hotline with the South used to co-ordinate traffic movements in and out of Kaesong, after previously cutting another hotline between the countries' Red Cross offices on March 11. Tensions have been rising since Pyongyang's third nuclear test in February led to a tightening of international sanctions against it. The isolated communist regime has said its recent threats against the South and the US were in response to the sanctions and to recent US-South Korean military drills. North Korea was the topic of a phone call on Wednesday between the defence chiefs of the US and China, North Korea's chief ally. Hagel told Chinese Defence Minister Chang Wanquan that the two nations should co-operate to halt North Korea's "growing threat," Pentagon press secretary George Little said. Unease and irritation toward North Korea has risen internationally. Australia has condemned its "belligerent behaviour" and said it was considering further sanctions against Pyongyang. Germany called for "utmost prudence" from both sides and urged China to use its influence to help quell North Korean aggression. Pyongyang said that it would restart a nuclear plant with the capacity to produce weapons-grade plutonium, after it shut down and partly demolished the facility in 2007 in return for promises of international aid.