Russian billionaire Roman Abramovich and at least two Ukrainian peace negotiators reportedly suffered symptoms of suspected poisoning earlier this month after a meeting in Kyiv.
The Wall Street Journal and the investigative outlet Bellingcat reported the claims, citing people familiar with the matter who laid the blame on hard-liners in Moscow wanting to sabotage peace talks with Ukraine.
Abramovich and at least two senior members of the Ukrainian negotiating team were affected, the WSJ report said.
However, Reuters quoted a US official, speaking on condition of anonymity, as saying intelligence suggested an "environmental" reason for the symptoms suffered by Abramovich and the negotiators, "Eg, not poisoning".
Here's what we know so far.
Roman Abramovich is very rich
Abramovich, who grew up a poor orphan in south-western Russia and served in the Soviet Union's army, was one of the most powerful businessmen who earned fabulous fortunes after the 1991 break-up of the USSR.
A commodity trader who thrived in the post-Soviet disorder under then-president Boris Yeltsin, he acquired stakes in the Sibneft oil company, Rusal aluminium producer and Aeroflot airline, and later sold them for massive profits.
Under Vladimir Putin, Abramovich served as governor of the remote Arctic region of Chukotka in Russia's Far East before buying Chelsea Football Club in 2003.
He is reportedly worth about $10 billion.
The West's reaction to the war in Ukraine, though, has hit him globally.
Abramovich has been sanctioned by Britain, the European Union and Canada, but not the United States, for his links to Vladimir Putin's regime.
The Premier League disqualified him from running Chelsea earlier this month.
He was in Kyiv at Ukraine's request
Abramovich was asked by Ukraine to help mediate with Moscow because of his background in Russia, a source with knowledge of the matter told Reuters late last month.
The BBC reported that Abramovich had been to Kyiv for several rounds of talks earlier this month and had met Ukrainian President Volodymyr Zelenskyy.
The Kremlin said Abramovich played an early role in peace talks between Russia and Ukraine but said the process was now in the hands of the two sides' negotiating teams.
Those teams are due to meet in Istanbul on Tuesday for the first face-to-face peace talks in more than two weeks.
The alleged poisoning victims suffered peeling skin
According to the WSJ report, Abramovich and the negotiators showed symptoms that included red, painful and constantly watering eyes, and peeling skin on their faces and hands, after the round of talks on March 3.
Abramovich and the Ukrainian negotiators, including Crimean Tatar MP Rustem Umerov, have since improved and their lives are not in danger, the WSJ reported.
A person familiar with the matter confirmed the details to Reuters but said Abramovich had carried on working.
It's not clear what poison may have been used
Bellingcat said experts who examined the incident concluded "poisoning with an undefined chemical weapon" was the most likely cause.
"An alternative less likely hypothesis was use of microwave irradiation," it said in a tweet.
The three men who experienced the symptoms had only consumed water and chocolate in the hours beforehand, Bellingcat said.
A fourth member of the team who also consumed these items did not experience symptoms, it said.
Citing the experts, Bellingcat said the dosage and type of toxin used was not enough to be life-threatening, "and most likely was intended to scare the victims as opposed to cause permanent damage".
"The victims said they were not aware of who might have had an interest in an attack."
Russia has form on this sort of thing
Moscow has been accused of using poison numerous times, going back decades.
In 2020, Russian opposition leader Alexei Navalny was allegedly poisoned by a Russian agent with a near-fatal dose of the Soviet-era Novichok nerve agent.
Novichok was also used in the 2018 attempted murder of former double agent Sergei Skripal, who sold Russian secrets to Britain, in the English city of Salisbury.
Russia has been accused of using other poisons to eliminate enemies too.
The European Court of Human Rights ruled Russia was responsible for the 2006 killing of ex-KGB officer Alexander Litvinenko, who died an agonising death after he was poisoned in London with polonium-210, a rare radioactive isotope.
The Ukrainians are dubious it was poison
Ukrainian officials have poured cold water on the report.
Asked about the suspected poisoning, Ukrainian negotiator Mykhailo Podolyak said "there is a lot of speculation, various conspiracy theories".
Mr Umerov urged people not to trust "unverified information".
Ukrainian Foreign Minister Dmytro Kuleba later took a similar line in an interview on national television, but also issued a warning.
"I advise anyone going for negotiations with the Russian Federation not to eat or drink anything and, preferably, avoid touching any surface," he said.
"That's at first hand.
"Secondly, all these stories — someone heard something somewhere, some sources — now there is a lot of this kind, because everyone is thirsty for news, for sensations."
Reuters said the Kremlin did not respond to a request for comment.
ABC/wires
