Ukraine's Yushchenko in hospital

Yushchenko, in front of his election poster, claims he was poisoned

Ukraine's opposition leader Viktor Yushchenko has gone into hospital in Austria for tests into the mystery illness that disfigured his face.

Doctors will spend the weekend examining him to assess whether his illness was caused by poisoning.

Mr Yushchenko was taken ill in September and resumed his presidential campaign with a pock-marked face - a marked difference in his appearance.

Fraud marred the poll so the second round is being re-run on 26 December.

The Supreme Court ordered the re-run after the opposition, backed by foreign observers, reported massive fraud in the 21 November vote.

Mr Yushchenko will again face Prime Minister Viktor Yanukovych, whose official win last time led to massive opposition protests on the streets of Kiev.

Wife 'tasted medicine'

Mr Yushchenko has alleged that his opponents poisoned him during September's campaign but doctors have not confirmed this.

At the clinic in Vienna, doctors will carry out tissue biopsies, including tests on Mr Yushchenko's skin.

I thought there was something different about my husband when he came home that night - because he has never taken any medicine Kateryna Yushchenko

"It will be an entire imaging diagnosis to look at the size and function of his various organs. We are going to reassess the entire blood chemistry, including possible types of poisoning," said Dr Michael Zimpfer, director of the Rudolfinerhaus hospital.

Dr Zimpfer said that if Mr Yushchenko's appearance was caused by chloracne - an acute form of acne which can be caused by dioxins - it could take two to three years to clear.

Mr Yushchenko's wife Kateryna told ABC's Good Morning America she tasted medicine on her husband's lips when she kissed him the night before he became ill.

"I thought there was something different about my husband when he came home that night - because he has never taken any medicine. He's a very healthy man," she said.

Arriving at the clinic on Friday, Mr Yushchenko told reporters he was getting "better health every day".

Earlier, both Mr Yushchenko and Mr Yanukovych were on the campaign trail.

The prime minister accused the Yushchenko camp of trying to intimidate his supporters in the western regions of Ukraine where the opposition had been the dominant force.

"The vote results won't be fair," Mr Yanukovych told reporters in Kiev.

Before leaving for Vienna, Mr Yushchenko predicted that he would win with more than 60% of votes. He vowed to serve the interests of the whole country, including supporters of Mr Yanukovych, whose stronghold is in the east and south.

In other developments:

• Mr Yanukovych's native Donetsk region postpones a self-rule referendum planned for 9 January

• Russian President Vladimir Putin says he has no objection to Ukraine joining the EU - in contrast to previous statements.

Opposition supporters have been scaling back their street protests after parliament agreed on Wednesday to reform the electoral law to avoid vote-rigging on 26 December.

Parliament also agreed to transfers some presidential powers to parliament.