David Cameron is "gravely concerned" at reports of Russian military vehicles crossing into Ukraine, his spokesperson said.
After speaking to Ukraine's President Petro Poroshenko, the Prime Minister said Russia must not use its humanitarian aid convoy as a pretext for provocation and asked Russia to "demonstrate willingness to find a peaceful solution to conflict in Ukraine."
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The FBI said three key elements of the Sony Pictures cyber attack led them to identify the administration in Pyongyang as the culprits.