Up to 35 rebels have been killed so far in fighting with Ukrainian forces in Donetsk, a pro-Russian separatist has claimed.
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Nearly 1,000 properties have been left without power, and road users have been warned of potentially hazardous conditions.
It follows a surprise summit between South Korea's Moon Jae-in and the North's Kim Jong-un.
Organisers issued a warning about a “dangerous high strength or bad batch substance on site”.