VILNIUS, LITHUANIA (TIP): Issuing an outright warning to Moscow, Vice President Joe Biden declared Wednesday the United States will respond to any aggression against its Nato allies, as Russia‘s neighbors looked warily to the escalating crisis in nearby Ukraine.

Standing side by side with a pair of Baltic leaders in Vilnius, Lithuania, Biden said the US was “absolutely committed” to defending its allies, adding that President Barack Obama plans to seek concrete commitments from Nato members to ensure the alliance can safeguard its collective security. In a jab at Russia, he said the US stands resolutely with Baltic states in support of the Ukrainian people against Russian aggression.

“Russia cannot escape the fact that the world is changing and rejecting outright their behavior,” Biden said, after meeting in Vilnius with Lithuanian President Dalia Grybauskaite and Latvian President Andris Berzins. Biden’s comments came at the close of a two-day trip to Lithuania and Poland with a two-pronged theme: Sending a stern message to Russian President Vladimir Putin that the US won’t abide Russian intervention in Ukraine, and reassuring fretful Nato allies that the US and others will come to their defense if necessary. “We’re in this with you, together,” Biden said.

Amid the tough talk from Biden and the Baltic leaders, Russia’s annexation of Crimea was increasingly looking like a foregone conclusion. At the Ukrainian navy headquarters in Sevastopol, Crimea, militias stormed the base Wednesday, taking it over without resistance. Although senior Ukrainian officials planned to travel to Crimea in hopes of averting an escalation in hostilities, Crimea’s pro-Russian prime minister insisted they weren’t welcome and wouldn’t be allowed to enter. A day earlier, Putin declared Crimea part of Russia in a passionate speech steeped in Russia’s sense of being slighted and marginalized by the West in the years since the Cold War.

While repeatedly insisting that Russia’s move is illegal and won’t be recognized, the US and other world powers have also turned their attention to eastern Ukraine and other areas with large ethnic Russian populations, lest Putin seek additional territory in what some fear could portend a return to Moscow’s traditional imperialist ambitions. To that end, Western powers were seeking fresh ways to show that Russia would incur real costs unless it changes course.

Berzins announced that he and Polish President Bronislaw Komorowski had agreed that Poland and Latvia will start coordinating its security activities more closely. France‘s foreign minister said leaders of the Group of Eight world powers have suspended Russia’s affiliation with the group over its actions in Ukraine. Obama invited the seven other members to discuss what comes next during an emergency meeting in Europe. Meanwhile, Britain said it was suspending military cooperation with Russia in light of the crisis. And German Chancellor Angela Merkel, in a phone call with Obama on Tuesday, agreed that U.N. and other international monitors must be sent in to other parts of Ukraine without delay.