Russian billionaire Roman Abramovich suffered symptoms of suspected poisoning at talks in Kyiv earlier this month, sources close to him say.
The Chelsea FC owner who has now recovered reportedly suffered sore eyes and skin peeling.
Two Ukrainian peace negotiators were also said to have been affected.
The Wall Street Journal reported claims the alleged poisoning was orchestrated by hardliners in Russia who wanted to sabotage the talks.
The conditions of Mr Abramovich and the Ukrainian negotiators, who include Ukrainian MP Rustem Umerov, have improved since the incident on 3 March, the paper quoted sources as saying.
A source close to Mr Abramovich told the BBC he had suffered symptoms of suspected poisoning. They said he had now recovered and was continuing with negotiations to try and end the war in Ukraine.
Earlier on Monday, Mr Umerov tweeted that he was “fine” and urged people not to trust unverified information.
The incident casts light on Mr Abramovich’s reported role as a broker in talks between
Ukraine and Russia. The exact nature of his position is unclear, but a spokesperson for the oligarch has previously said his influence is limited.
On Sunday, President Zelensky said Mr Abramovich had offered him help to de-escalate Russia’s invasion of the country.
The Russian billionaire travelled between Moscow and Kyiv for several rounds of talks at the start of the month.
He reportedly met President Volodymyr Zelensky during the trip, but the Ukrainian leader was not affected and his spokesman had no information about the incident.
The investigative journalism group Bellingcat said Mr Abramovich and the negotiators suffered symptoms “consistent with poisoning with chemical weapons”.
The symptoms included “eye and skin inflammation and piercing pain in the eyes, Bellingcat reported.
Meanwhile, an unnamed US official was quoted by Reuters as saying that intelligence suggested the sickening of the men was due to environmental factors, not poisoning.
Mr Abramovich has since been seen in public, being pictured at Israel’s Tel Aviv airport on 14 March.
Mr Abramovich was sanctioned by the EU and UK earlier this month over his alleged links to Russia’s President Putin, which he denies.
But Mr Zelensky has reportedly asked the US to hold off from sanctioning 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 peace talks but the process was now in the hands of the two countries’ negotiating teams.
The two sides are set to meet in Istanbul on Tuesday for their first face-to-face peace talks in more than two weeks.
On the evening of 3 March, Roman Abramovich and Ukrainian peace negotiators met in Kyiv, right in the heart of Ukrainian territory. What happened next is very mysterious.
Later that evening, three of them, according to the investigative website Bellingcat, suffered symptoms of nerve agent poising.
They had inflamed skin, irritated eyes and severe pain behind the eyes – symptoms which lasted all night.
None of them had eaten anything more, according to Bellingcat, than chocolate and water.
Chemical weapons specialists have examined this case and concluded that they believe it was intentional.
But we have no idea who did it. There is no claim of responsibility.
Inevitably people will be wondering if this was the work of the GRU, Russia’s military intelligence service, who Britain concluded did the Novichok Salisbury poisoning.
There is no word from Russia and no proof as to who was behind it.
Somebody, it seems, wanted to send a warning to those taking part in the peace talks.
This was not a lethal dose, it was a warning.
The suggestion made by an unnamed US official that environmental factors were to blame is odd.
Nobody else was affected by these very serious medical afflictions which Bellingcat’s investigators concluded were the result of an intentional use of a chemical agent.
It would hardly be surprising that the US would want to dampen down suggestions that anyone especially Russia had used a chemical weapon in Ukraine, as this could push them into retaliatory action that they are extremely reluctant to take.