Polls have opened in South Korea's presidential elections with the pro-government candidate, Roh Moo-hyun, still reeling from the last minute defection of a crucial supporter.

The popular football chief Chung Mong-joon, who had stepped down as a presidential candidate last month to clear the way for Mr Roh, said he was withdrawing his support for the human rights lawyer.

Mr Chung Mong-joon, head of National Alliance 21, has decided to withdraw his support for candidate Roh National Alliance 21 statement

A statement from Mr Chung's party, National Alliance 21, said the decision had been taken over remarks made by Mr Roh about a possible war between North Korea and the United States.

The withdrawal could prove a heavy blow for Mr Roh, who went into Thursday's election neck-and-neck with his conservative rival, Lee Hoi-chang.

Mr Chung's support has been deemed crucial.

The wealthy US-educated son of a conglomerate founder, is seen to have gained in popularity and influence since South Korea's success in co-hosting, and competing in, the World Cup finals earlier this year.

'Intervention'

Candidates used their final hours of campaigning to hammer home their different messages, in a race which has been overshadowed by the threat posed by North Korea.

Roh Moo-hyun

Former human rights lawyer Favours engaging with North Korea Popular with younger voters

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Mr Roh, of the pro-government Ruling Millennium Party, has been pledging to continue outgoing President Kim Dae-jung's so-called "sunshine policy" of engaging with the secretive North, warning that any change in policy would lead to "tension and Cold War hostility".

National Alliance 21 said that Mr Roh, in a speech on Wednesday night, suggested that South Korea would have to intervene if the US started a war with North Korea.

"The United States is our ally," said Mr Chung in the statement. The remark, he added "is extremely inappropriate and runs against the spirit of the alliance of the two parties."

Since withdrawing his own candidacy, Mr Chung had spent the last few weeks campaigning on Mr Roh's behalf.

Nuclear allegations

Mr Roh's rival Lee Hoi-chang, of the opposition Grand National Party, believes dialogue with the North should be frozen until Pyongyang dismantles its suspected nuclear programme.

"The clearest distinction between myself and candidate Roh is that I believe the 'sunshine policy' is a failure," Mr Lee said.

Which of the two men wins could have important implications for relations on the tense Korean peninsula.

Lee Hoi-chang

High Court judge known as "Mr Clean" Hardline on North Korea Lost 1997 presidential race to Kim Dae-jung

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The result will be closely watched by the United States.

President George W Bush has labelled North Korea part of an "axis of evil" and backs the same hard-line policies favoured by Mr Lee.

The threat posed by North Korea was brought home to voters last week when Pyongyang said it was resuming a nuclear programme frozen under a 1994 deal.

Although it is not clear whether North Korea issued the warning as a threat or bargaining chip, there are widespread fears that the programme could be used to produce nuclear weapons.

Too close to call

The winner will formally take office in February, when President Kim, who is constitutionally barred from running again, steps down.

Analysts said a recent wave of anti-American sentiment in South Korea could also be a factor in the election.

Thousands of people have marched in protest at the acquittal by a US court martial of two US soldiers whose armoured vehicle ran over and killed two Korean schoolgirls in June.

It is unclear whether the public anger has damaged the campaign of Mr Lee, who is perceived as a friend of the US, or whether it will boost the chances of Mr Roh.

On Wednesday, an independent candidate, Chang Se-tong, dropped out of the election race, leaving six contenders.

Mr Chang said it would be meaningless to remain in the election as Mr Roh and Mr Lee are battling for first place.