Brno/Prague - Czech citizen Ivo Stejskal was killed in the fighting in Ukraine, his sister Monika Stejskalova told public broadcaster Czech Television (CT) today.
"I don´t want to provide any information. He´s dead," she told CT.
Czech media speculate about another Czech victim, Vojtech Hlinka, who allegedly fought with the pro-Russian rebels along with Stejskal.
But Czech authorities have not confirmed the death of either of the men.
Foreign Ministry spokesman David Frous said the Czech Republic has received no official reports on deaths of Czechs in Ukraine. The state does not follow private activities of individuals, however, he added.
According to information Stejskalova posted on Facebook, Stejskal and Hlinka died in Ukraine on August 12.
CT reported that Stejskal, a former primary school teacher of physical training and civics, left for Ukraine in June to join the separatists.
Stejskal, aged 40, was a rent defaulter, CT said.
Several Czechs probably joined the separatists based on recruitment campaigns on social networking websites, CT reported.
Under Czech legislation, military service in foreign armies is banned and may be punished with up to five years in prison, except for deployment in armies of NATO members and military service approved by the president.