Russia has said it is "no longer prepared to tolerate" any delays to its aid convoy entering Ukraine.
In a statement the foreign ministry said: "All excuses to delay sending aid have been exhausted. The Russian side has decided to act."
Reuters witnesses reported the trucks are still currently at the border but Russia warned against any attempts to disrupt them coming through.
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Among those expected to attend the ceremony will be former US President Bill Clinton and ex-Northern Ireland first minister Peter Robinson.
Five people have been killed and 40 injured after a car and knife attack in London on Wednesday.
A wet, wintry night in places with rain and hill snow slow to move away from southern Scotland and northern England.