The Wall Street Journal and the investigative website Blingcat cited sources that Russian billionaire Roman Abramovich, who is under Western sanctions, and Ukrainian negotiators developed poison-like symptoms after a meeting in Kyiv this month.
The Wall Street Journal reported that Abramovich, who accepted a request from Ukraine to help negotiate an end to the Russian invasion, and at least two senior members of Ukraine's negotiating team were affected by the incident.
The sources indicated that symptoms, including redness of the eyes and peeling of the skin of the face and hand, appeared on them, pointing out that Abramovich did not allow the accident to prevent him from working.
The newspaper confirmed that Abramovich and the Ukrainian negotiators improved after that and that their lives were not in danger.
The newspaper quoted a source close to Abramovich as saying that it was not yet clear who targeted the group.
She added that the group, which suffered from symptoms, blamed what it described as hardliners in Moscow aiming to sabotage the talks.
It cited Western experts as saying that it was difficult to determine whether the symptoms were caused by a chemical or biological agent, or by an electromagnetic radiation attack.
Ukraine's presidential adviser, Mykhailo Podolyak, did not confirm the incident, instead advising that "only official information" should be pursued.
All members of the negotiating team are working as usual today, Podolyak said, stressing that there has been a lot of speculation about information in the media and various conspiracy theories.
chemical weapon
As for the Blingcat website, it said that experts who examined the incident concluded that the most likely cause was "poisoning by an unspecified chemical weapon."
The site quoted experts as saying that the dose and type of poison used were not enough to threaten life, and that "the aim was likely to frighten the victims and not to do permanent harm to them. The victims said they did not know who might have an interest in carrying out an attack."
Blingcat confirmed on Twitter that the three men who had symptoms had only eaten chocolate and water in the hours before symptoms appeared.
The Kremlin said Abramovich played a role in the early stages of peace talks between Russia and Ukraine, but the process is now in the hands of the two negotiating teams on both sides.
For his part, Ukrainian President Volodymyr Zelensky confirmed on Sunday that his government had received offers of support from Russian businessmen, including Abramovich, who is seeking to sell Chelsea Football Club and has a close relationship with Putin.
Zelensky told reporters that these Russian businessmen said they wanted "to do something and help" to stop the escalation of Russia's military offensive on Ukraine "which has claimed thousands of lives".
The Wall Street Journal reported last week that Zelensky had asked US President Joe Biden to delay imposing sanctions on Abramovich, because the Russian billionaire could play a role in negotiating a peace deal with Moscow.
Russian forces invaded Ukraine on February 24, while President Vladimir Putin described the war as a "special military operation" to disarm Ukraine, while Ukraine and the West say Putin launched an unjustified war of aggression.
It is scheduled to meet the Russian and Ukrainian sides on Tuesday in the Turkish city of Istanbul in direct talks for the first time in more than two weeks.