KIEV, July 8 - Russia has refused to provide its samples of dioxin, the highly toxic substance used to poison then-opposition leader Viktor Yushchenko in September 2004, two months before he won the presidency, General Prosecutor Oleksandr Medvedko said Friday.

"The Russian party simply does not respond," Medvedko said. "They keep silence and that's it."

Toxicology experts said three countries—the US, Britain and Russia--could actually produce the sophisticated brand of dioxin used to poison Yushchenko. The U.S. and Britain have already provided their samples, Medvedko said.

The samples, once checked with those found in Yushchenko's body, could show exactly where the dioxin had been produced.

The fact that the investigation has been stalled despite the arrival of dioxin samples from the U.S. and Britain perhaps suggests those samples didn't match those found in Yushchenko's body, analysts said.

Russia's refusal to cooperate in one of Ukraine's most high profile investigations raises questions of whether the Russian-produced dioxin was actually used in the poisoning, analysts said.

Russia, led by President Vladimir Putin, took unprecedented measures in 2004 to try to prevent the election of Yushchenko, a pro-Western leader, to the presidency in Ukraine.

Putin arrived twice in Ukraine during the election campaign to publicly endorse Yushchenko's rival, Prime Minister Viktor Yanukovych, a pro-Russian figure. Putin also twice officially congratulated Yanukovych with "the victory" after the vote had been blatantly rigged in his favor, as had been later shown by a court ruling.

Yushchenko seeks to speed up Ukraine's accession to NATO and the European Union, but Russia has been desperately seeking to prevent that scenario in order to maintain its influence on the former Soviet Union.

Yushchenko, then a presidential candidate, first fell ill after having dinner with Ukrainian Security Service chief Ihor Smeshko and his deputy, Volodymyr Satsyuk, on Sept. 5, 2004. He reported having a headache about three hours after the dinner, and by the next day had developed an acute stomach ache.

Yushchenko, who was rushed to a Vienna hospital on Sept. 10, later reported pancreatitis and gastrointestinal pain, as well as backache. He also suffered partial nerve paralysis in his face and an inflammation of one inner ear.

About three weeks after his first symptoms, he developed the rough, acne-like rash on his face which is the hallmark of dioxin poisoning.

Satsyuk, who hosted the dinner, later escaped to Russia, which has refused to extradite him.

But Yushchenko said that he was confident that the investigation will eventually lead to those who had been involved in the poisoning.

"The investigation uncovered details, such as how did the [dioxin] dosage been prepared, how it was served, how did it come to Yushchenko, what time there
was, what food products," Yushchenko said in an interview with Inter television.

"Now, there are couple of scoundrels that must be caught and prosecuted by law," Yushchenko said. "They will be caught. I have no doubts." (tl/ez)