Japan's coast guard said on Monday the North has sent out signals warning ships against entering the area for the next three weeks.

The coast guard picked up a similar warning in May days before Pyongyang conducted a second nuclear test and later launched a series of short-range missiles.

Heightened tensions

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The warnings and weapons tests have ramped up tensions on the Korean peninsula with South Korean and US forces on high alert.

The North's latest warning also came on the same day that a Pyongyang court sentenced US journalists Laura Ling and Euna Lee to 12 years hard labour after convicting them of entering the country illegally.

Supporters and families of the two reporters fear they have become caught up as pawns in the bigger stand-off between North Korea on one side and the US and its allies on the other.

On Sunday Hillary Clinton, the US secretary of state, warned that the Obama administration was considering returning North Korea to its blacklist of state sponsors of terrorism.

North Korea was removed from the list in October last year as part of a six-nation aid for disarmament deal.

But with North Korea announcing last month that it was abandoning the disbarment talks that deal now appears to have been nullified.

Japan, which has been arguing for a tougher international approach to North Korea, has said it hopes North Korea will be relisted by the as sponsor of terrorism.

"When North Korea was pulled off the list, Japan issued a protest with the United States," Takeo Kawamura, a senior spokesman for the Japanese government, told reporters in Tokyo.

"We have to continue putting pressure on North Korea."

'Unacceptable'

South Korean troops are on high alert following the latest warnings from the North [EPA]
On Sunday Japan's foreign minister Hirofumi Nakasone pressed the case for tougher action during talks with his Chinese counterpart, Yang Jiechi, calling for Beijing to support a "strong" UN resolution to convey to the North the message that a nuclear test was unacceptable.

The official said Nakasone told Yang that "China's role as a regional power and a country with close ties with North Korea is great in this issue".

Yang's response was that China would support the swift passing of a resolution that is "appropriate and balanced", the official told reporters in Tokyo.

"China's position is quite clear," Yang was quoted as telling Nakasone.

"China is firmly against North Korea's nuclear experiments or holding nuclear [weapons]. That is the same as what the foreign ministry has said in a statement."