The G7 has issued a formal statement urging Russia to back away from its efforts to split the Crimea from Ukraine.
The group, which includes some of the world's top economies, says a Russian-backed referendum on the status of the Black Sea peninsula would have no legal effect.
The statement also says the referendum process is deeply flawed, given its rushed nature and the intimidating presence of Russians troops in the Crimea.
"We call on the Russian Federation to immediately to halt actions supporting a referendum on the territory of Crimea regarding its status, in direct violation of the Constitution of Ukraine," it says.
"Any such referendum would have no legal effect. Given the lack of adequate preparation and the intimidating presence of Russian troops, it would also be a deeply flawed process which would have no moral force. For all these reasons, we would not recognize the outcome."
The G7 countries -- Canada, France, Germany, Italy, Japan, the United Kingdom and the United States -- were joined in the statement by the president of the European Council and the president of the European Commission.
They say a Russian annexation of the Crimea would violate a number of international agreements.
"In addition to its impact on the unity, sovereignty and territorial integrity of Ukraine, the annexation of Crimea could have grave implications for the legal order that protects the unity and sovereignty of all states," the statement says.
"Should the Russian Federation take such a step, we will take further action, individually and collectively."
The statement urges Russia to withdraw its troops and offers to mediate talks between Moscow and the Ukrainian government.
NATO, meanwhile, is flexing its muscles in the region.
The military alliance has deployed two surveillance planes to monitor Ukraine's air space as well as ship traffic in the Black Sea as Russia's military buildup in Crimea was continuing.
A NATO spokesman says the two aircraft will monitor Russian movements from inside Poland and Romania -- two NATO members that border Ukraine.