US Vice President Joe Biden on Tuesday warned Russia it faced the prospect of more sanctions over its move to absorb of Crimea, calling it nothing more than a "land grab".
"Russia's political and economic isolation will only increase if it continues down this path and it will in fact see additional sanctions by the United States and the EU," Biden said during a visit to Warsaw, after the Kremlin announced it now considers Crimea a part of Russia.
"Russia has offered a variety of arguments to justify what is nothing more than a land grab," Biden told reporters after talks on security cooperation with Poland's Prime Minister Donald Tusk.
Britain and Germany strongly condemned Moscow's inking of a treaty making Crimea a Russian territory, with British Foreign Secretary William Hague saying London was suspending all bilateral military cooperation.
The events in Crimea have rattled nerves in Poland and the three Baltic states, which were under Moscow's thumb before the collapse of the Soviet Union in 1991.
Biden was in Poland to reassure regional allies as Russia tightened its grip on Ukraine's breakaway region.
At a press conference with Tusk he condemned what he termed "steps to annex Crimea" by Russian President Vladimir Putin.
Biden accused Russia of responding to the democratic aspirations of the Ukrainian people with "a brazen military incursion with the purposeful ratcheting up of ethnic tensions inside Ukraine".
"With a rushed and illegal referendum in Crimea that was not surprisingly rejected by virtually the entire world. And now today with steps to annex Crimea," he said.
"We've joined Poland and the international community in condemning the continuing assault on Ukraine's sovereignty and territorial integrity and the blatant violation of international law by Mr. Putin in Russia."
Biden will hold talks with Polish President Bronislaw Komorowski as well as the presidents of Estonia, Latvia and Lithuania -- meeting the latter two in Vilnius on Wednesday -- to reassure the ex-communist NATO allies of Washington's commitment to mutual defence under Article 5.
However, he will "not discuss any changes to the missile defence strategy in eastern Europe... designed to respond to a ballistic missile threat from elsewhere, not Russia," a senior White House official aboard Air Force One said earlier.
Earlier, Poland's Komorowski and Lithuania's President Dalia Grybauskiate discussed stepping up security in the region.
"The heads of state acknowledge that Russia's actions in Ukraine and demonstration of military power in (the Russian exclave of) Kaliningrad pose threats to the security of the Baltic region and Poland," Grybauskaite said in a statement after the telephone talks.
"Therefore, immediate measures and concerted efforts are needed to strengthen regional security," she said.
NATO has already ramped up security in its ex-communist member states bordering Ukraine and Russia.
The United States last week sent six additional F-15 fighter jets to step up NATO's air patrols over the Baltics.
NATO has deployed AWACS reconnaissance aircraft to overfly Poland and Romania as part of its efforts to monitor the crisis in Ukraine.
Washington is also sending a dozen F-16 fighter jets and 300 service personnel to Poland as part of a training exercise.