Ukraine sent a large armoured column and special forces towards the flashpoint city of Slavyansk today as Kiev's fledgeling leaders tried to reassert control over the separatist east.
The seven buses, filled with 100 paratroopers, and 20 tanks and armoured personnel carriers represented the Western-backed team's most forceful response to date to raids that have seen state buildings in nearly 10 cities across Ukraine's rust belt fall under the control of pro-Russian militants.
They had moved within 40 kilometres of Slavyansk and set up a checkpoint that could control all traffic leading to Slavyansk -- an economically depressed industrial city of 100,000 that has been under effective control of separatist gunmen since Saturday.
"They must be warned that if they do not lay down their arms, they will be destroyed," Ukrainian Security Service (SBU) General Valeriy Krutov told a group of reporters tracking the sudden tank movements.
The SBU anti-terrorism centre director alleged that the gunmen had received reinforced by a deployment of several hundred soldiers from the Russian army's Main Intelligence Directorate (GRU).
Increasingly insistent Western charges of the Kremlin's direct involvement in the spreading eastern unrest set a chilly tone to a "frank and direct" exchange on the crisis between US President Barack Obama and his Russian counterpart Vladimir Putin.
But yesterday's heated phone conversation appeared to break no new ground. The Kremlin chief continued to reject any links to Russian-speaking gunmen who have proclaimed the creation of their own independent republic and who have called on Putin to send in the estimated 40,000 Russian troops now stationed along the border with Ukraine.
European foreign ministers meanwhile held back on unleashing punishing economic sanctions against Russia in hopes that EU-US mediated talks on Thursday in Geneva between Moscow and Kiev could help de-escalate the most dire East-West standoff since the Cold War.