US Air Force General Philip Breedlove, Nato's Supreme Allied Commander Europe, said Russia was acting more like an adversary than a partner and the 28-nation alliance should rethink the positioning and readiness of its forces in eastern Europe.
Russian troops seized some of the last military facilities under Ukrainian control on Saturday in Crimea, the Black Sea peninsula that Russian President Vladimir Putin formally annexed the day before.
General Breedlove was one of several Western officials and politicians to warn that Russia may not stop there in a crisis that has taken East-West relations lurching back towards the Cold War since pro-Western protests in Ukraine ousted Moscow-allied President Viktor Yanukovich last month.
"The (Russian) force that is at the Ukrainian border now to the east is very, very sizeable and very, very ready," the Nato commander told an event held by the German Marshall Fund think-tank.
US President Barack Obama's deputy national security adviser Tony Blinken said the build-up might just be aimed at intimidating Ukraine's new pro-Western leaders but that Russia could invade the country's mainly Russian-speaking east.
Russia said it was complying with international agreements and had no plans to invade. It has called the soldiers who took over Ukrainian bases in Crimea "self defence forces".
The US and the EU have targeted some of Mr Putin's closest political and business allies with personal sanctions and have threatened broader economic sanctions if Mr Putin's forces encroach on other eastern or southern parts of Ukraine that have big Russian-speaking populations.
Mr Blinken said Washington was considering all requests for military assistance from the government in Kiev, but that it would be unlikely to prevent an invasion of Ukraine, which is not part of Nato. He said the military alliance needed to think about its eastern members, particularly the former Soviet Baltic states of Lithuania, Latvia and Estonia.
Mr Breedlove said Nato was very concerned about the threat to Transdniestria, which declared independence from Moldova in 1990 but has not been recognised by any UN member state. About a third of its half million population is ethnic Russian.
Moscow's ambassador to the EU, Vladimir Chizhov, said Russia did not have "expansionist views". Asked to give a commitment that Russian troops would not move into Ukrainian territory outside Crimea, he said: "There is no intention to do anything like that."
But US Republican Senator John McCain said Mr Putin's actions in Ukraine were akin to those of Adolf Hitler in 1930s Germany.
"I think he (Mr Putin) is calculating how much he can get away with, just as Adolf Hitler calculated how much he could get away with in the 1930s," he said.
Germany's Foreign Minister Frank-Walter Steinmeier underscored the huge potential repercussions of Russia's bid to redraw national borders in Europe. "I'm very worried the unlawful attempt to alter recognised borders in our European neighbourhood, 25 years after the end of the Cold War, will open Pandora's Box," he said.
Last night, Mr Putin and German Chancellor Angela Merkel spoke by telephone and voiced satisfaction an agreement had been reached to send monitors from the Organisation for Security and Cooperation in Europe to Ukraine
Belarussian President Alexander Lukashenko, a close ally of Russia, has accepted Crimea is now "de facto" a part of Russia, but said the annexation set a "bad precedent".
President Obama and US Secretary of State John Kerry are due in The Hague today to meet Russian Foreign Minister Sergei Lavrov for bilateral talks.