Bosworth's comments came on the same day that North Korea warned it would use its weapons if provoked – an apparent effort to deter international punishment for its test of a nuclear device last month.

Nuclear 'offensive'

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"Our nuclear deterrent will be a strong defensive means ... as well as a merciless offensive means to deal a just retaliatory strike to those who touch the country's dignity and sovereignty even a bit," said a commentary in the state-run Minju Joson newspaper.

It appeared to be the first time that North Korea referred to its nuclear arsenal as "offensive" in nature. Pyongyang has long claimed that its nuclear weapons programme is a deterrent and only used for self-defence.

The tough talk came as South Korea and the US lead an effort at the United Nations Security Council to have the North punished for last month's nuclear test with increased sanctions.

According to the Seoul-based Yonhap news agency, China – North Korea's closest ally - has agreed on a draft resolution on further sanctions against North Korea for the test.

"China has agreed on a compromise draft resolution presented by the US and the Western countries," a diplomat told the agency, adding that an announcement on fresh measures against the North could be made within a couple of days.

The unnamed diplomat said that Russia has yet to agree to the draft, citing the need to consult officials in Moscow.

The five veto-wielding permanent members of the UN Security Council, plus South Korea and Japan, have met several times since the North's May 25 nuclear test, but have failed to smooth over differences over measures to be adopted in response.

On Wednesday South Korea's chief envoy to talks on North Korea's nuclear programme travelled to Beijing for talks with Chinese officials on the response to the North's recent nuclear and missile tests.

Families 'devastated'

There are fears the US reporters jailed in North Korea could be used as bargaining chips [AFP]
As the diplomatic wrangling continues, the families of two US journalists sentenced to 12 years hard labour in North Korea have appealed to the government in Pyongyang for leniency saying they are "shocked and devastated'' by the sentence.

Laura Ling and Euna Lee were jailed on Monday after being found guilty of crossing illegally into North Korean territory.

The two women, who were detained by North Korean border guards on March 17, were working for California-based Current TV, an online and television service set up by Al Gore, the former US vice-president.

While Washington has labelled the case of the journalists as a "humanitarian matter," analysts expect the North will link the issue to negotiations on the country's disputed nuclear programme.