South Koreans braced for the ''very high'' chance of a North Korean missile test as a top American military commander said the totalitarian state had moved at least one projectile to its eastern coast.
The possibility of a ballistic missile launch was ''very high'' and ''may materialise any time from now'', South Korean Foreign Minister Yun Byung-se told lawmakers in Seoul on Wednesday.
South Korean and US forces upgraded their joint surveillance ''Watchcon'' status to monitor an imminent missile fire, Yonhap News Agency reported, citing unnamed military officials.
On the move: South Korean soldiers near the border city of Paju. Photo: AFP
Japan has deployed interceptors around the country as Prime Minister Shinzo Abe's government vowed to protect its citizens.
Admiral Samuel Locklear, head of US Pacific Command, confirmed to the US Congress that North Korea moved at least one of its medium-range Musudan missiles to its eastern coast. Efforts by Kim Jong-un's regime to build and test weapons of mass destruction ''represent a clear and direct threat to US national security and regional peace and stability'', he said.
Admiral Locklear said Mr Kim, the third-generation head of North Korea, is ''more unpredictable'' than his late father and grandfather. Kim Jong-il and Kim Il-sung always figured into their ''provocation cycle an off-ramp of how to get out of it. And it's not clear to me that he has thought through how to get out of it,'' he said.
In recent weeks, Mr Kim's regime has threatened to launch pre-emptive nuclear strikes against its enemies, and this week pulled workers from a joint industrial complex with South Korea.
More than 53,300 North Korean labourers again did not show up for work at the Gaeseong industrial complex on Wednesday, extending for a second day the suspension of operations for 123 South Korean companies there, Unification Ministry spokesman Kim Hyung-suk said in Seoul. Of 406 South Koreans in Gaeseong, 115 were scheduled to leave the park on Wednesday.
Tensions have risen since North Korea's February atomic weapons test in defiance of tightened United Nations sanctions that were backed by China, its closest ally and biggest trading partner.
A warning that foreigners in South Korea could be in danger, and should prepare for evacuation in case of war, came from the North's official Korean Central News Agency on Tuesday. The statement was dismissed as ''psychological warfare'' by a spokeswoman for South Korean President Park Geun-hye.
The North Korean leader's conduct was an attempt to gain attention and concessions for his impoverished state rather than a sign that war was imminent, according to Huh Moon-young, an analyst at the state-run Korea Institute for National Unification in Seoul.
''North Korea is not looking to self-destruct,'' Mr Huh said. ''It's trying to raise an issue with the international community and also grab the US and China's attention in a highly calculated manner.''
The American mission in Seoul had seen no unusual military movements suggesting preparations for war, and the US was ''not encouraging Americans to take any special travel preparations'', US State Department spokesman Patrick Ventrell said on Tuesday.
Any weapons test may coincide with the April 15 anniversary of the birth of Kim Il-sung, the leader's grandfather.
The Musudan missile's range is enough to make it a potential threat to Guam, though not to Hawaii or the US mainland, Admiral Locklear told the Senate armed services committee in Washington on Tuesday.