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Ukrainian rebels' request for military assistance from Moscow represents an “additional step against” the country's sovereignty, the EU said on Wednesday.
Earlier in the day, the Kremlin said the heads of the separatist republics had requested “help” to “repel Ukraine's aggression”, a move that opens the door for massed Russian troops to move in.
“The EU strongly urges Russia to refrain from any further escalatory actions,” said Josep Borrell, the EU's foreign affairs chief.
The rebels had written letters to Russian President Vladimir Putin, according to Kremlin spokesman Dmitry Peskov.
Their appeals came after Mr Putin recognised the separatist regions' independence and signed friendship treaties that included defence deals.
Washington and Britain say Russia is poised to strike Ukraine and trigger the most serious war in Europe for decades, but Mr Putin says he is open to negotiation – within limits.
EU leaders are due to meet on Thursday evening in Brussels for a summit to discuss the “latest developments” following Moscow's recognition of the separatist Ukrainian regions and the bloc's adoption of economic sanctions.
The meeting will also discuss ways to “hold Russia accountable” and support Ukraine.
