SEOUL, April 1, 2008 (AFP) - North Korea on Tuesday launched a barrage of insults against South Korea's new president in its first comments since he took office, saying his harder line on relations could have 'catastrophic consequences.'

The lengthy and vitriolic attack on Lee Myung-Bak -- which describes the conservative South Korean leader as a US sycophant, a traitor and a charlatan -- comes as tensions are rising between the two nations.

On Sunday the hardline communist state's official media claimed that Seoul was planning a preemptive military strike and threatened to turn South Korea into 'ashes' if it went ahead.

In recent days the North has also expelled South Korean officials from a joint industrial complex, test-fired missiles, accused Seoul of breaching a disputed sea border and threatened to suspend all dialogue.

Media reports say the North's jet fighters have increased sorties near the border.

Lee, who took office February 25, has pledged a tougher line against North Korea, including linking economic aid to the impoverished nation to its nuclear disarmament.

Tuesday's commentary in the communist party newspaper Rodong Sinmun described Lee's policy as a 'declaration of war', and also blasted his intention to press the North on its human rights record.

'The Lee regime will be held fully accountable for the irrevocable catastrophic consequences to be entailed by the freezing of inter-Korean relations and the disturbance of peace and stability on the Korean Peninsula due to its sycophancy towards the US and its moves for confrontation with the north,' it said.

A South Korean presidential official who declined to be identified described the attack on Lee, carried by the official Korean Central News Agency, as 'not proper.'

US Assistant Secretary of State Christopher Hill, who arrived in Seoul for talks on ways to end a stalemate in six-party nuclear negotiations, played down the comments.

He described them as 'completely inappropriate' but said they were part of domestic propaganda.

'I think we should probably not over-react to comments that really have no basis in fact and seem to be entirely propagandistic and aimed at domestic audiences, whoever they are,' Hill told reporters.

Hill, the chief US negotiator in the nuclear talks, said he did not expect the North's recent rhetoric -- such as a threat to reduce the South to 'ashes' -- would have a significant impact on the six-way process.

'I don't think I will take a fire extinguisher with me to the next set of talks,' he said.

Analysts say the North may be trying to sway the South's April 9 election. Lee's Grand National Party wants to win a parliamentary majority over its liberal rivals, who practised a decade-long 'sunshine' engagement policy towards Pyongyang.

The North may also want to undermine Lee before his first summit with US President George W. Bush this month, analysts say.

Rodong Sinmun insisted that denuclearisation is not a bilateral issue.

Lee's 'call on 'the north to dismantle its nukes first' is nothing but a declaration of confrontation and a war declaration as it throws a hurdle in the way of the settlement of the nuclear issue...' it said.

North Korea last year pledged to disable its atomic plants and declare all nuclear programmes and materials by the end of 2007.

The North says it submitted the declaration last November. The US says it has not fully accounted for a suspected secret uranium enrichment programme and for allegations of nuclear proliferation to Syria.

The commentary blasted Lee's 'impertinent' actions in urging the North to open up its society and improve human rights. It ridiculed his pledge to raise the North's per capita income to 3,000 dollars within a decade if it fully denuclearises.

'This has brought to light his true colours as a vicious political charlatan and impostor,' it declared.

The North accepted economic and food aid, and investment worth billions of dollars under Seoul's previous policy. But the party newspaper said it does not need Seoul's help.

'The DPRK will be able to live as well as it wishes without any help from the South, as it did in the past,' it said, using the North's official name the Democratic People's Republic of Korea.