Russian billionaire Roman Abramovich suffered symptoms of suspected poisoning at peace talks on the Ukraine-Belarus border earlier this month, sources close to him say.
The Chelsea FC owner - who has now recovered - reportedly suffered sore eyes and peeling skin.Two Ukrainian peace negotiators were also said to have been affected.
One report said the alleged poisoning was orchestrated by hardliners in Russia who wanted to sabotage talks.
Shortly after the allegations emerged, an unnamed US official was quoted by Reuters as saying that intelligence suggested the men's symptoms were due to "environmental" factors, not poisoning.
And later an official in the Ukrainian president's office, Ihor Zhovkva, told the BBC that while he hadn't spoken to Mr Abramovich, members of the Ukrainian delegation were "fine" and one had said the story was "false".