The Ukrainian president has again accused Russian army officers of fighting alongside pro-Moscow separatists in the east of the country.
Petro Poroshenko said: "Russian staff officers are taking part in military operations against Ukrainian forces."
The war of words between Kiev and Moscow followed a weekend of intense fighting, in which Ukrainian forces say they inflicted heavy losses on pro-Russian rebels.
More top news
A hacker group who claimed to be behind a cyber attack on Sony Pictures have posted a message online apparently mocking the FBI.
The returns of Benedict Cumberbatch's detective and the X Factor judge were among 2014's most tweeted TV moments in a billion conversations.
A family has issued an appeal to help find their missing pet turkey - in case he gets gobbled up as someone's Christmas dinner.