According to sources close to Roman Abramovich, he experienced symptoms of suspected poisoning while attending peace talks on the Ukraine-Belarus border earlier this month.
The Chelsea FC owner reportedly suffered from stinging eyes and peeling skin, but has since recovered.
According to reports, two Ukrainian peace negotiators were also harmed.
According to one story, the alleged poisoning was planned by Russian hardliners who intended to derail the discussions.
Shortly after the charges surfaced, Reuters quoted an unidentified US official as saying that intelligence indicated the men's illnesses were caused by "environmental" reasons rather than poisoning.
Later, Ihor Zhovkva, a spokesman for Ukraine's president, told the BBC that while he hadn't spoken to Mr Abramovich, members of the Ukrainian delegation were "fine" and one had stated the news was "fake."
However, as BBC security journalist Frank Gardner points out, it's not surprising that the US would seek to downplay claims that anyone - particularly Russia - used a chemical weapon in Ukraine, as this may lead to retaliatory action that the US is hesitant to do.
According to the Wall Street Journal, Mr. Abramovich's and the Ukrainian negotiators' conditions have improved since the March 3 incident, which included Ukrainian MP Rustem Umerov.
According to a source close to Mr Abramovich, he has recovered and is continuing to negotiate to stop the crisis in Ukraine.
The event sheds light on Mr. Abramovich's alleged role as a middleman in Ukraine-Russia discussions. His specific role is unknown, but a spokesman for the oligarch earlier stated that his influence was "limited."
Ukraine's President, Volodymyr Zelensky, announced on Sunday that Mr. Abramovich had offered him assistance in de-escalating Russia's invasion of the country.
At the beginning of the month, the Russian billionaire traveled between Moscow and Kyiv for multiple rounds of negotiations. During the trip, he apparently saw Mr Zelensky, but the Ukrainian leader was unaffected, and his spokeswoman had no idea about it.
The investigative journalism group Bellingcat, meanwhile, 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.
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 Vladimir 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.
Roman Abramovich met with Russian and Ukrainian peace negotiators on the Ukraine-Belarus border on March 3 in the afternoon. What transpired afterwards is a complete mystery.
According to the investigative website Bellingcat, three of the delegates, including Mr Abramovich, developed signs of nerve agent poisoning later that night.
Their skin was inflamed, their eyes were irritated, and they had significant pain behind their eyes that lasted all night.
According to Bellingcat, none of them had consumed anything other than chocolate and water.
