A missile launch by North Korea would be "very unhelpful" to moving forward its relationship with the United States, U.S. Secretary of State Hillary Clinton said on Tuesday.

Speaking a day after Pyongyang said it had the right to launch a long-range missile, Clinton said Washington was watching North Korea very closely to see if it ends its provocative action.

"The possible missile launch that North Korea is talking about would be very unhelpful," she told a news conference in Tokyo after talks with her Japanese counterpart.

South Korean media reports say Pyongyang has been preparing to test its longest-range Taepodong-2 missile, which is designed to reach as far as Alaska but has never successfully flown.

Officials in South Korea have also been anticipating a short-range missile test near a disputed sea border where navies from the two countries have clashed in the past.

Clinton repeated the offer she made in a speech last Friday of a peace treaty -- which included the normalisation of ties and aid -- if North Korea eliminated its nuclear weapons programme, the topic of long-running six-country talks.

There has been no response yet from Pyongyang to the offer.

"If North Korea abides by the obligations it has already entered into and verifiably and completely eliminates its nuclear programme, then there will be a reciprocal response, certainly from the United States," she said.

"But the decision as to whether North Korea will cooperate in the six-party talks, end provocative language and actions, is up to them -- and we are watching very closely."

In recent weeks, North Korea's harsh rhetoric has increased sharply, including a threat to destroy the wealthy South, in anger at the hardline policies of its President Lee Myung-bak.

Analysts say North Korea is using the missile threat to put pressure on President Lee to end curbs he placed on aid to the destitute state and grab the Obama administration's attention.

They say Pyongyang may feel it can extract more from the international community by raising tensions rather than by accepting Washington's olive branch.

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