North Korea appears to be readying for some kind of a rocket launch, a U.S. defense official said Thursday, though it did not appear to be a ballistic missile.

The official's comments came after Japanese media reported that satellite imagery showed Pyongyang seemed to be preparing a long-range ballistic missile launch from the Dongchang-ri site in North Korea's west.

The United States is aware of reports that a U.S. citizen has been detained in North Korea, the State Department said Friday, after Pyongyang declared it had arrested an American student for an unspecified "hostile act".

"The welfare of U.S. citizens is one of the department's highest priorities," U.S. State Department spokesman John Kirby said in Davos, Switzerland.

North Korea's leadership should be held criminally responsible for egregious human rights abuses, a U.N. envoy said Friday.

The United Nations has slammed the isolated nation for rights violations, detailing what it described as horrific abuses, including state-sponsored abductions, in a 2014 report.

North Korea's leader Kim Jong-Un is afraid of sharing the fate of slain dictators Saddam Hussein, Moammar Gadhafi and Al-Qaida's Osama bin Laden, and is working on a nuclear deterrent for this purpose, a former Japanese defense minister said Friday.

"Kim is very afraid to be killed like (Iraq's) Saddam Hussein, (Libya's) Moammar Gadhafi or Osama bin Laden," Satoshi Morimoto, who is now a national security expert, told a conference in Athens.

The U.S. sent a heavy bomber over South Korea on Sunday in a show of force as North Korean leader Kim Jong-Un insisted his country's latest nuclear test was carried out in self-defense.

The test on Wednesday of what the North claimed was its first hydrogen bomb has sparked international alarm and raised tensions along the inter-Korean frontier, with Seoul reviving cross-border propaganda broadcasts.

North Korea's leader Kim Jong-Un on Sunday justified what he claimed was his country's first hydrogen bomb test as self-defense to prevent nuclear war with the U.S., in his first comments since the explosion.

Pyongyang on Wednesday carried out its fourth nuclear test, angering the international community and raising tensions with neighboring South Korea.

South Korea on Friday resumed high-decibel propaganda broadcasts into North Korea as the United States ramped up pressure on China to bring Pyongyang to heel after its latest nuclear test.

While North Korean leader Kim Jong-Un celebrated his 32nd birthday, the international community scrambled to find common ground on how best to penalize his regime following its shock announcement two days ago that it had successfully tested its first hydrogen bomb.

Sanctions alone will not change North Korea's behavior after this week's nuclear test and China needs encouragement -- not threats -- to alter its dealings with its renegade neighbor, a U.S. expert said Thursday.

China's role in influencing North Korea is again in the spotlight following Pyongyang's fourth nuclear test. Beijing is the country's main ally and several figures including Republican presidential hopeful Donald Trump have demanded China do more.

Britain summoned the North Korean ambassador in London on Thursday, the Foreign Office said, after Pyongyang's latest nuclear test drew international condemnation.

"I summoned North Korea's Ambassador today to stress in the strongest terms the UK's condemnation of their nuclear test," Asia minister Hugo Swire said in a Foreign Office statement.

With a surprise nuclear test two days before his birthday, North Korea's young leader Kim Jong-Un has once again asserted his personal control over the hermit state he inherited from his late father four years ago.

When he came to power after Kim Jong-Il's death in December 2011, the younger Kim was considered untested, vulnerable and likely to be manipulated by senior figures.