Kiev sharply raised the stakes in Europe's most explosive crisis in decades by declaring yesterday that a Ukrainian transport plane downed in the eastern conflict zone had been hit by a rocket fired from the Russian side of the frontier between the two ex-Soviet states.
Russia had not publically answered the charge by this morning. A Kremlin spokesman had earlier dismissed a Moscow newspaper report of Russia weighing up "targeted retaliatory strikes" against Ukraine as "complete nonsense".
A top Ukrainian general went a step further late yesterday by telling a live television audience in Kiev that he feared a Russian invasion was imminent.
"Ukraine, like never before, stands on the cusp of a wide-scale aggression from our current northern border," National Security and Defence Council Deputy Secretary Mykhaylo Koval said on private ICTV television.
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Koval said the Kremlin had parked 22,000 troops in the annexed Black Sea peninsula of Crimea and had other units stretching from the north-central region of Chernigiv to the southeastern edge of the Russian-Ukranian border on the Sea of Azov.
Putin rejects accusations of orchestrating the uprising in reprisal for the February ouster of a Russian-backed leader and Kiev's subsequent signature of an historic EU alliance instead of a new Kremlin pact.
But Poroshenko argues that no truce with the rebels is possible until his troops manage to seal the Russian border and halt the continuing flow of gunmen and arms.
The frontier became the conflict's new frontline after last week's evacuation by the rebels of a host of towns and cities that they had held since early April in the coal mining region of Donetsk.