Pro-Russian rebels have said the death toll during two days of fighting in the eastern Ukraine city of Donetsk has now risen to 50.
The prime minister of the rebels' self-styled Donetsk People's Republic (DNR) Alexander Borodai told Russia's RIA news agency: "We have lost more than 50 volunteers."
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Mladic, now 75, was the military commander of Bosnian Serb forces in the 1990s, who fought against the Bosnian Croat and Bosniak armies.
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His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."