One of the eight European observers being detained by pro-Russian separatists in the eastern Ukraine city of Slaviansk was escorted to an Organisation for Security and Cooperation in Europe (OSCE) vehicle and driven away, according to Reuters.
Stella Khorosheva, a spokeswoman for the separatist mayor of Slaviansk, said the observer who left is a Swedish national. "He has a mild form of diabetes and so we decided to let him go," she told reporters.
According to the spokeswoman, he would be the only observer to be released on Sunday.
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Cloud and outbreaks of rain will ease away eastwards during the morning, clearing the east coast around midday with sunny spells developing.
Police and emergency services are attending the scene.
Rebel-held east remains at risk while the pumping station supplying the western part of the city is once again in operation.