The Ukrainian prosecutor general has said Russia is not co-operating with his inquiry into the poisoning of President Victor Yushchenko in 2004.
Oleksandr Medvedko said that despite two requests, Moscow had not provided a sample of dioxin, the poison given to Mr Yushchenko before he was elected.
The highly dangerous toxin left his face badly disfigured and pockmarked.
An investigation into the attempted murder of Mr Yushchenko was opened, but no-one has ever been charged.
Mr Yushchenko's pro-Western allies have accused Russia of being behind the attempt to kill him, a charge Moscow denies.
'No explanations'
Tests in late 2004 revealed that pure TCDD, the most harmful known dioxin, had been used on Mr Yushchenko.
At a press conference in the Ukrainian capital, Kiev, Mr Medvedko said only three countries in the world produced the chemical.
"The Russian side is giving no explanations - they are keeping silent"
He said a sample could point to the origin of the poison and that he had therefore asked for dioxin samples from these places.
Mr Medvedko said his office had received samples from the UK and USA, but not yet from Russia.
"We sent two letters asking for legal assistance in this sense, but there has been no positive result yet," he said.
"The Russian side is giving no explanations. They are keeping silent."
Mr Medvedko said he hoped to press the matter by holding talks with his Russian counterpart in the near future.
Mr Yushchenko was poisoned shortly before he swept to power in the 2004 liberal Orange Revolution, which overturned Prime Minister Viktor Yanukovych's allegedly fraudulent election victory.
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