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 its annexation.
The interim leaders in Kiev fear that Russian President Vladimir Putin -- flush with expansionist fervour -- 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 Secretary of State John Kerry will meet Russian Foreign Minister Sergei Lavrov with Russia facing the loss of its coveted seat among the G8 group of leading nations.
It will be their first meeting since Washington imposed financial restrictions on the most powerful members of Putin's inner circle for their decision to resort to force in response to last month's fall of Ukraine's pro-Kremlin regime.
- Transdniestr games 'worrisome' -
Foreign Minister Andriy Deshchytsya reaffirmed Ukrainian concerns in an interview broadcast on Sunday on a top US political talk show.
"We do not know what Putin has in his mind and what would be his decision. That's why this situation is becoming even more explosive than it used to be a week ago," Deshchytsya told ABC's "This Week".
NATO's Breedlove meanwhile said Russian military exercises in Transdniestr, which lies on Ukraine's southwestern border, were "worrisome".
"There is absolutely sufficient force postured on the eastern border of Ukraine to run to Transdniestr if the decision was made (in Moscow) to do that," Breedlove said, adding: "And that's very worrisome."
Hours later, however, Moscow issued an apparently conciliatory statement saying Putin had spoken to German Chancellor Angela Merkel and expressed "satisfaction" over the decision to send Organisation for Security and Co-operation in Europe (OSCE) monitors to heavily Russified southeastern Ukraine.
One of the biggest tests facing the besieged interim leaders in Kiev now comes from restless Russians who have been stirring up violent protests and demanding their own secession referendums in southeastern Ukraine.
Many in the region, whose cultural and trade ties with Russia go back centuries, mistrust the new leadership's European values.
On Sunday Crimea's Russia-backed prime minister Sergei Aksyonov said the peninsula began facing a "sad fate" the moment three months of deadly protests toppled the pro-Kremlin regime in Kiev.
"But we resisted and won! Our motherland -- Russia -- extended her hand of help," said Aksyonov. "So today, I appeal to you with a call to fight."
Aksyonov said he was "deeply convinced" that the future of southeastern Ukraine "rested in a close union with the Russian Federation -- a political, economic and cultural union".
- Stun grenades -
Crimea's authorities estimate they together with the Kremlin's forces control at least half of Ukrainian bases on the Black Sea peninsula and about a third of its functioning naval vessels.
Ukraine's acting defence minister Igor Tenyukh on Sunday lamented that his navy officers appeared too ready to surrender to Kremlin-backed militias and Russia's Black Sea Fleet that has made Crimea its home since the 18th century.
"You know that in recent days, we have had our ships blockaded and seized even though our commanders had the authorisation to use force," Tenyukh told reporters in Kiev.
"Unfortunately, the commanders made decisions on the spot. They chose not to use their weapons in order to avoid bloodshed."
The Ukrainians' refusal to engage Russian forces has led to a domino-like fall of their bases across the rugged peninsula of two million people.
The most dramatic episode of Russia's excursion so far saw crack forces on Saturday break into the Belbek airbase near the main city of Simferopol after an armoured personnel carrier blasted through the main gate.
Two more armoured personnel carriers followed and gunmen stormed in firing automatic weapons into the air.
Ukraine's interim President Oleksandr Turchynov said Sunday that the Russian forces had captured the base commander.
- Bid to 'splinter Europe' -
Russia's diplomatic isolation is now growing as quickly as the reemergence of an ideological divide that appeared to have been bridged with the 1989 fall of the Berlin Wall.
Germany's foreign minister warned after talks with Ukraine's leaders that the continent's future was at stake.
The show of diplomatic solidarity may play an important psychological role in Kiev as it faces new pressure from Russia that includes open threats to throw Ukraine's wheezing economy into convulsion by raising its gas rates and demanding colossal payments for disputed debts.
Yet both the United States and Europe have thus far limited their retaliation against Putin to targeted travel and financial sanctions that concern officials but do not impact the broader Russian economy.
Washington's steps have been more meaningful because they hit what US officials call a Putin "crony bank" as well as oligarchs who are believed to be closest to the Russian strongman.