Experts trying to determine why Ukrainian opposition leader Viktor Yushchenko (search) fell seriously ill during the presidential election campaign are testing several poisoning theories, but there's no conclusive evidence about what caused the disease, doctors said Wednesday.

Yushchenko's face has been disfigured, with ashen, pockmarked skin, since he fell ill in September. He has claimed that Ukrainian authorities poisoned him — an allegation they deny.

Doctors are still running tests to try to determine what caused his ailments, said Dr. Michael Zimpfer (search), director of Rudolfinerhaus, the private Vienna clinic where Yushchenko was treated.

"At the present stage, we are still investigating the hypothesis of poisoning," Zimpfer said. "However, we have not found any indication that a chemical or biological substance has been employed. Also, we are following new threads, and (we have) included other labs to do more specific testing."

Yushchenko suffered from a series of symptoms, including back pain, acute pancreatitis and nerve paralysis on the left side of his face.

"He was very ill but there was no immediate danger to his life," Zimpfer told a news conference.

Doctors have only "a descriptive diagnosis" but no proof of what led to the ailments, Zimpfer said, adding they could have had internal causes, or have been sparked by a poison.

"It might also have been a combination of poisons. Everything is in the air," he said.

Making the mystery even more difficult to solve was Yushchenko's refusal to let doctors take biopsies of his facial tissue — he reportedly said he did not want to have his face bandaged while campaigning — and the four-day delay between the outbreak of the ailments and his arrival at the hospital, Zimpfer said. Doctors later received some tissue samples, he added.

The London newspaper The Times on Wednesday quoted Dr. Nikolai Korpan, the Rudolfinerhaus physician who oversaw Yushchenko's treatment, as saying that Yushchenko had been poisoned and the intention was to kill the candidate.

However, Korpan, appearing at a news conference Wednesday with Zimpfer, said doctors were merely working on three different poisoning theories, including one involving dioxin.

"At this point, we don't have any evidence of what caused the illness," he said.

He later acknowledged having spoken to The Times but declined to discuss what he said.

Yushchenko first sought treatment at Vienna's private Rudolfinerhaus clinic on Sept. 10. He went home in mid-September to resume campaigning, but came back to the hospital later that month for more treatment.

Before Yushchenko was released in early October, doctors fit an epidural in his upper back, between the shoulder blades, to numb overpowering back pain, Zimpfer said, adding he traveled with Yushchenko to Ukraine to ensure that no problems would arise.

"It's a tricky procedure," Zimpfer said. "It's dangerous. But it worked very fine."

One of the chief doctors treating Yushchenko, Lothar Wicke, was been placed under police protection after receiving an anonymous threat. No details about the threat have been released.

Prime Minister Viktor Yanukovych (search) officially defeated Yushchenko in the Nov. 21 presidential election runoff, but the Supreme Court canceled the results as fraudulent and ordered a rematch for later this month.