Russian billionaire Roman Abramovich and Ukrainian peace negotiators Abramovich, who accepted a Ukrainian request to help negotiate an end to the Russian invasion of Ukraine, and at least two senior members of the Ukrainian team, were affected, the WSJ report said.
Ukrainian officials 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".
Rustem Umerov, another member of the negotiating team, urged people not to trust "unverified information".
Ukrainian Foreign Minister Dmytro Kuleba later took a similar line, saying in a television interview that "everyone is thirsty for news and sensations".
However, he also added wryly: "I advise anyone going for negotiations with Russia not to eat or drink anything (and) preferably avoid touching surfaces".
A US official said intelligence suggested an "environmental" reason for the sickening of Abramovich and the negotiators, "E.g., not poisoning".
The official spoke on condition of anonymity and did not elaborate further.
The Kremlin did not immediately respond to an emailed request for comment.
According to the WSJ report, Abramovich and the negotiators showed symptoms that included red eyes, constant and painful tearing, and peeling skin on their faces and hands.
Abramovich and the Ukrainian negotiators, including Crimean Tatar lawmaker Umerov, have since improved and their lives are not in danger, WSJ reported.
A person familiar with the matter confirmed the incident to Reuters but said Abramovich had not allowed it to stop him working.
The Press Association in the UK cited sources as saying Abramovich had now recovered and was continuing to try to help with negotiations.
It said he had been involved in talks about securing humanitarian corridors to allow Ukrainians to leave as well as bringing other countries to the negotiating table.
Bellingcat said experts who examined the incident concluded "poisoning with an undefined chemical weapon" was the most likely cause.
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."
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.
Russian forces invaded Ukraine on February 24 in what President Vladimir Putin calls a "special military operation" to demilitarise Ukraine.
Ukraine says Putin launched an unprovoked war of aggression.
The Kremlin has said Abramovich played an early role in peace talks between Russia and Ukraine but the process was now in the hands of the two sides' negotiating teams.
The two sides are due to meet in Istanbul on Tuesday for the first face-to-face peace talks in more than two weeks.