"We are faced with phenomena that we haven't experienced before," he said.
Storm Thalia hit several regions of mainland Greece on Saturday, but the weather was improving on Sunday.
Local officials in Evia said two rivers had burst their banks and filled roads with mud.
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Seven people killed as flash floods hit Greek island
A baby was among seven people who died as torrential rains and floods swept the Greek island of Evia, also known as Euboea, damaging dozens of houses and blocking roads.
https://www.rte.ie/news/2020/0809/1158232-greece-floods-deaths/