EU calls snap summit on Russia-Ukraine crisis
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"It is important that we continue to be united and determined and jointly define our collective approach and actions," he said in the letter published on Wednesday.
"Therefore, I would like to invite you for a special European Council (summit) on Thursday 24 February, which will take place in person in Brussels and start at 20:00 (1900 GMT)."
The European Union says it is keeping more, tougher sanctions up its sleeve should the Russian forces invade further, beyond the territory that the separatists hold.
"The use of force and coercion to change borders has no place in the 21st century," Michel wrote in his summit invitation.
"The aggressive actions by the Russian Federation violate international law and the territorial integrity and sovereignty of Ukraine. They also undermine the European security order," he said.