NATO and Poland both warned that Russia may be gearing up to invade its former Soviet neighbour, despite strong denials from Moscow.
The first air strike on rebel-held bastion Donetsk came as Ukrainian forces said they were preparing to liberate the city, although they also reported their highest death toll in weeks in the face of fierce rebel bombardments.
Today, NATO warned that Russia had increased the number of "combat-ready" troops on its border with Ukraine to 20,000 from 12,000 in mid-July.
Poland's Prime Minister Donald Tusk said "the threat of a direct intervention (by Russia into Ukraine) is certainly greater than it was even a few days ago."
Moscow said those making the claims about their troop movements were "selling soap bubbles".
"Movements of such forces of thousands of troops and equipment are not possible in such a short time," said Russian defence ministry spokesman Igor Konashenikov.
Against this backdrop, Kiev said NATO chief Anders Fogh Rasmussen will visit tomorrow at the invitation of President Petro Poroshenko.
The West accuses Russia of supporting and instigating the insurgency in eastern Ukraine, sending tensions with Moscow soaring to their highest point since the Cold War.
The US and European Union have slapped a range of tough sanctions on Moscow, with Switzerland, Japan and Canada following suit, potentially pushing Russia's fragile economy towards recession.
But Russian President Vladimir Putin struck back today with import bans and restrictions on agricultural products from countries that have targeted Moscow.
