Vadym Boichenko said the attack occurred on Monday, 11 April.
The mayor of besieged Mariupol has claimed Russian planes dropped a chemical agent in an area on the outskirts of the city.
Vadym Boichenko, speaking at an online press conference on Wednesday, said the attack occurred on Monday, 11 April.
"They sensed a sweet taste in their mouths in the towns and villages around Mariupol," said Boichenko, referring to the locals affected. "They started fleeing because they sensed danger."
Euronews cannot independently verify these claims.
Mariupol City Council said on its Telegram channel that the victims of the "poisonous attack" were in a satisfactory condition.
In its post, one of the alleged victims, a Ukrainian soldier, said the attack happened in a sparsely populated area.
The council said an elderly woman was the worst-affected victim.
Symptoms of the attack included "facial flushing, high blood pressure, dryness and heartburn in the oropharynx, mucous membranes of the eyes", the council added.