Sources close to him say Russian billionaire Roman Abramovich was found to have suspected poisoning during peace talks along the Ukraine-Belarus border earlier this month.
The owner of Chelsea FC - who has now recovered - reportedly suffered from eye pain and peeling skin.
Two Ukrainian peace negotiators are also reported to be affected.
A report said the alleged poison was created by Russian hardliners who wanted to sabotage the talks.
Shortly after the allegations surfaced, an unnamed US official was quoted by Reuters as saying that intelligence suggested the men's symptoms were due to "environmental" factors, not poison.
And Ihor Zhovkova, an official in Ukraine's presidential office, later told the BBC that while he did not speak to Mr Abramovich, members of the Ukrainian delegation were "fine" and that one had said the story was "false".
However, BBC security correspondent Frank Gardner says it is hardly surprising that the US would want to downplay suggestions that someone - notably Russia - had used a chemical weapon in Ukraine, as it would allow them to retaliate. I can push that they are extremely reluctant to take.
A source close to Mr Abramovich told the BBC he had recovered and was continuing talks to try to end the war in Ukraine.
The incident highlights Mr. Abramovich's alleged role as a broker in talks between Ukraine and Russia. The exact nature of his position is unclear, but a spokesman for the elite has previously said his influence was "limited".
On Sunday, Ukrainian President Volodymyr Zelensky said Mr Abramovich had offered him help to reduce Russia's invasion of the country.
The Russian billionaire traveled between Moscow and Kyiv earlier in the month for several rounds of talks. He reportedly met Mr. Zelensky during the visit, but the Ukrainian leader was not impressed and his spokesman had no knowledge of the incident.
Meanwhile, investigative journalism group Bellingcat said Mr Abramovich and the interlocutors were experiencing symptoms "consistent with poisoning with chemical weapons".
Bellingcat reports that symptoms include "eye and skin inflammation and eye pain."
Mr Abramovich has since been seen in public, being pictured at Israel's Tel Aviv airport on March 14.
Mr Abramovich was banned earlier this month by the European Union and Britain because of his alleged links with Russian President Vladimir Putin, which he denies.
But Mr Zelensky has reportedly asked the US to stop approving Mr Abramovich, arguing he could play a role in negotiating a peace deal with Moscow.
The Kremlin has said Mr Abramovich played an early role in the peace talks but the process was now in the hands of the negotiating teams of the two countries.
The two sides are due to meet in Istanbul on Tuesday for the first face-to-face peace talks in more than two weeks.
On the afternoon of March 3, Roman Abramovich joined Russian and Ukrainian peace negotiators in talks on the Ukraine-Belarus border. What happened after that is very mysterious.
Later that night, three delegates - including Mr. Abramovich - suffered symptoms of nerve agent poisoning, according to investigative website Bellingcat.
He had skin swelling, burning eyes and severe pain behind the eyes – symptoms that lasted all night.
According to Bellingcat, none of them had eaten anything more than chocolate and water.
Chemical weapons experts investigated the matter and concluded that they believed it was the intentional use of a chemical agent.
But we don't know who did it. No claim of responsibility.
Inevitably people may be wondering whether this was the work of the GRU, Russia's military intelligence service, which Britain concluded was behind Novichok's Salisbury poisoning in 2018.
There was no immediate comment from Russia and no evidence that they were responsible.
But it seems that someone wanted to warn the participants in the peace talks. It was not a lethal dose, it was a warning.
The suggestion by an unnamed US official that environmental factors are to blame is strange.
No one else was affected by these serious medical hardships. Chemical weapons expert, Hamish de Breton-Gordon, told the BBC it was highly unlikely that environmental factors had anything to do with it.