U.N. chief Ban Ki-moon said Thursday he was "deeply concerned" by rising tensions on the Korean peninsula after Pyongyang said it had approved plans for nuclear strikes on US targets.

Speaking on a visit to Monaco, Ban also said he hoped North Korea would "as soon as possible" lift restrictions on South Korean workers at the Kaesong complex, the last real surviving point of contact between the two countries.

North Korea must stop stoking tensions and instead re-engage with the international community, the European Union said Thursday after Pyongyang warned it is ready to launch a nuclear attack on the United States.

EU foreign affairs chief Catherine Ashton said she deplored Pyongyang's announcement it would reopen its mothballed Yongbyon reactor which supplied plutonium for its weapons program.

The United States has scrambled to reinforce its Pacific missile defenses, preparing to send ground-based interceptors to Guam, as North Korea said Thursday it had authorized plans for nuclear strikes on U.S. targets.

U.S. Defense Secretary Chuck Hagel said Pyongyang's increasingly bellicose threats combined with its military capabilities represented a "real and clear danger" to the United States and to its allies South Korea and Japan.

North Korea's threats and recent actions represent a "real and clear danger" to the United States as well as its allies South Korea and Japan, U.S. Defense Secretary Chuck Hagel said Wednesday.

"They have nuclear capacity now, they have missile delivery capacity now," Hagel said after giving a major strategy speech at the National Defense University, calling Pyongyang's "bellicose dangerous rhetoric" problematic.

There is no consensus on an international arms trade treaty, major weapons exporter China said Wednesday, after it abstained as the U.N. General Assembly passed the measure by 154 votes to three.

The only votes against the first treaty on the conventional arms trade were from Syria, North Korea and Iran.

North Korea blocked access to a key joint industrial zone with South Korea on Wednesday -- a sharp escalation in a military crisis that Washington blamed on Pyongyang's "reckless" behavior.

North Korea informed Seoul in the morning that it was stopping the daily movement of South Koreans into the Seoul-funded Kaesong complex -- 10 kilometers (six miles) inside the North side of the border.

U.N. chief Ban Ki-moon warned Tuesday that the Korean peninsula crisis could spiral out of control, after North Korea announced it would restart a nuclear reactor to feed its atomic weapons program.

"Nuclear threats are not a game," Ban said, responding to a series of aggressive statements by Pyongyang that have prompted the deployment of nuclear-capable U.S. B-52s, B-2 stealth bombers and a U.S. destroyer to South Korea.

North Korea vowed Tuesday to restart all mothballed facilities at its main Yongbyon nuclear complex, adding to tensions already raised by near daily warlike threats against the United States and South Korea.

The reactor was shut down in 2007 as part of international nuclear disarmament talks that have since stalled.

The United States has placed a destroyer off the South Korean coast to defend against a possible missile strike, the latest in a series of publicized U.S. deployments to counter North Korean threats.

The USS Fitzgerald was moved to the southwestern coast after taking part in annual military exercises, instead of returning to its home port in Japan, a U.S. defense official told Agence France Presse Monday on condition of anonymity.

The White House said Monday that despite days of bellicose rhetoric, North Korea had yet to back up its threats with mass troop mobilizations or troop movements.

With tensions on the Korean peninsula rising ever higher, Washington reiterated that it took Pyongyang's war talk seriously but also noted that threats and warnings were nothing new from the isolated state.