KIEV - Ukraine's Western-backed leaders voiced fears of an imminent Russian invasion of the industrial heartland on Sunday, as NATO's top commander warned of a "very sizeable" Russian troop presence on Ukraine's eastern border.
The warnings came a day after Kremlin troops seized Ukraine's last airbase in Crimea, deploying armoured personnel carriers and stun grenades in a spectacular show of force after sealing the peninsula's annexation.
The interim leaders in Kiev fear that Russian President Vladimir Putin is developing a sense of impunity after being hit by only limited EU and US sanctions for taking the Black Sea cape.
"The aim of Putin is not Crimea but all of Ukraine.... His troops massed at the border are ready to attack at any moment," Ukraine's National Security and Defence Council chief Andriy Parubiy told a mass unity rally in Kiev.
NATO's top commander, General Philip Breedlove, warned that the Russian force on the border was "very, very sizeable and very, very ready" and could threaten Transdniestr, a Moscow-backed separatist territory of Moldova.
Alarm about a push outside Crimea by Moscow's overwhelming forces - now conducting drills at Ukraine's eastern gate - were fanned further by a call from its self-declared premier for Russians across the ex-Soviet country to rise up against Kiev's rule.
Europe's most explosive crisis in decades will dominate a nuclear security summit opening in The Hague on Monday.
US President Barack Obama is to attend the gathering, on the first leg of a European trip that would also take in Brussels and the Vatican, before he continues on to Saudi Arabia.
US Secretary of State John Kerry will also meet Russian Foreign Minister Sergei Lavrov with Russia facing the loss of its coveted seat among the G8 group of leading nations.