Doctors treating Ukrainian opposition leader Viktor Yushchenko say they are looking at a highly toxic chemical as the cause of his mysterious illness.

Leading Austrian physician Michael Zimpfer told VOA, from a Vienna hospital, that specialists have narrowed the causes of Mr. Yushchenko illness to the presence of dioxin in his tissue. He said German and French scientists are helping with the investigation.

"It is extremely tricky to analyze, but we are now specifically searching the possibility of dioxin in very specialized labs and within the next couple of days we are getting new information," Dr. Zimpfer explained. "We sent the specimens to different labs both in the [United] States and in continental Europe. The Ukrainian government asked the Austrian government for support and assistance in order to either confirm or rule out the hypothesis of poisoning."

But Dr. Nikolai Korpan, who treated Mr. Yushchenko at the Rudolfinerhaus hospital last September, denied to VOA a report in the London daily, The Times, that quoted him as saying the Ukrainian opposition leader had been poisoned.

Mr. Yushchenko has accused the Ukrainian authorities of poisoning him, but officials in Kiev deny the charge.

Mr. Yushchenko was treated at the clinic during his presidential campaign for a mysterious illness that left his face disfigured by cysts and lesions.

Scientists say dioxin is a highly toxic chemical that was a primary component in Agent Orange, a defoliant used by the United States in Vietnam that caused illness among war veterans and civilians. Exposure to the chemical can result in cancer and various nerve and blood disorders.

One doctor who treated Mr. Yushchenko has reportedly received death threats and has special police protection. Mr. Yushchenko's health records have been confiscated by the courts in Vienna to assist Ukrainian security forces in their inquiries.