Ukraine has rejected a Russian proposal on how to determine the origin of the poison that sickened President Viktor Yushchenko three years ago, officials said Wednesday, according to Associated Press (AP),  Kiev, Ukraine.

Yushchenko, who was a leader of the political opposition at the time, was poisoned with dioxin during the 2004 presidential election campaign, disfiguring his face.

No arrests have been made, but suspicions of Russian involvement persist -both because Yushchenko was running against a Kremlin-backed candidate and because Russia is one of four countries that produces the specific formula of dioxin used against him.

Although the dioxin from all four countries is chemically identical, differences in the manufacturing process yield various byproducts; testing samples from each country for these byproducts could determine the origin of the dioxin found in Yushchenko.

Three countries that produce this type of dioxin -U.K., Canada and the U.S. -have submitted samples to Ukraine for testing, investigators say, but Russia has refused. This month it offered to test samples of its dioxin in Russia and report the results to Ukraine.

The Ukrainian Prosecutor General`s office rejected that offer this week, and again asked Russia to let the test be conducted in Ukraine, a spokesman said.

"Under Ukrainian law, the tests will only be valid if they are conducted on Ukrainian territory," spokesman Yuriy Boychenko said.

Yushchenko has complained that Russia was stalling the investigation by refusing to provide the dioxin samples and hand over key suspects. Ukrainian authorities have not named any suspects, but Yushchenko has said the suspects are hiding out in Russia.

On Wednesday, he said prosecutors would travel soon to Russia to meet with their counterparts, a visit he said he hoped would solve the stalemate.

"Maybe in the course of that dialogue negotiations will be concluded concerning the tests of dioxin which is produced in the Russian Federation and holding accountable those people who are hiding in the Russian Federation," he told reporters.

The Kremlin strongly backed Yushchenko`s rival, Viktor Yanukovych, in the bitterly contested 2004 presidential election, which deepened rifts between Moscow and the West. Yanukovych was initially declared the winner. Massive street protests -dubbed the Orange Revolution -broke out, and the Supreme Court threw out the results on grounds of fraud. Yushchenko won a court-ordered repeat vote.

Yushchenko has hinted that he knows those responsible for the poisoning. While refraining from naming the alleged culprits until the investigation is over, he has hinted that the poisoning could have been masterminded from outside the country.

This news was monitored by the Action Ukraine Monitoring Service for the Action Ukraine Report (AUR), Morgan Williams, SigmaBleyzer, Editor.