Vienna: What ails Viktor Yushchenko? As Ukraine's popular opposition leader claimed victory, the mystery surrounding an appearance-altering condition that twice prompted him to check into a Vienna hospital persisted.

Mr Yushchenko has accused the Ukrainian authorities of poisoning him. His detractors suggested he had eaten bad sushi. Adding to the intrigue, the Austrian doctors who treated him have asked foreign experts to help determine if his symptoms may have been caused by toxins found in biological weapons.

Medical experts say they may never know what befell Mr Yushchenko. But the condition has dramatically changed his appearance since he entered Vienna's private Rudolfinerhaus clinic on September 10.

Mr Yushchenko was known for his ruggedly handsome, almost movie-star looks. Now his complexion is pockmarked and a sickly green. His face is haggard, swollen and partially paralysed. One eye often tears up.

"It's becoming a puzzle," said Dr Marc Siegel, an associate professor at New York University's School of Medicine who has studied the case. "The longer it goes on, the less I think of food poisoning."

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John Henry, a toxicologist at Imperial College London, said photographs indicate Mr Yushchenko may have a condition known as chloracne - a type of adult acne caused by exposure to toxic chemicals.

"There aren't really very many other explanations. You don't just get this horrible acne-like illness out of the blue in a middle-aged man," Dr Henry said.

Steroid treatment or mercury poisoning could cause similar- looking acne, he said, but the greenish tinge of Mr Yushchenko's face is more suggestive of dioxin poisoning.