President Barack Obama warned Thursday that a referendum in Crimea on joining Russia would violate Ukraine's constitution and international law.
The president spoke hours after the United States imposed visa bans on certain senior Russian officials and moved towards wider sanctions against individuals and entities in Moscow, to punish the Kremlin's incursion into Ukraine.
"The proposed referendum on the future of Crimea would violate the Ukrainian constitution and violate international law," Obama told reporters at the White House.
"Any discussion about the future of Ukraine must include the legitimate government of Ukraine," Obama said.
Earlier, the parliament in Crimea, under the de facto control of pro-Russian forces since the ousting of Ukrainian president Viktor Yanukovych, asked Russian President Vladimir Putin to examine a request for their region to join the Russian Federation.
"In 2014, we are well beyond the days when borders can be redrawn over the heads of democratic leaders," the US president said.
Obama also said, on a day that the European Union also readied sanctions against Russia, that the world was united in its opposition to Russia's action and in its support for Ukraine.
But he also argued there remained a way out for Russia, as talks continue between Secretary of State John Kerry and his Russian counterpart Sergei Lavrov in the Europe.
Such a deal would see the world support elections in Ukraine in May, to allow international monitors into Ukraine and for Russian forces to keep their Black Sea bases, he said.
"But if this violation of international law continues, the resolve of the United States and our allies and the international community will remain firm," Obama said.
The President also on Thursday ordered the freezing of U.S. assets and a ban on travel into the United States of those involved in the Russian military intervention into the Crimea region of Ukraine.
Obama signed an executive order aimed at punishing those Russians and Ukrainians responsible for a Russian move into Crimea, a crisis that has raised old-style Cold War tensions.
The order, the White House said in a statement, is "a flexible tool that will allow us to sanction those who are most directly involved in destabilizing Ukraine, including the military intervention in Crimea, and does not preclude further steps should the situation deteriorate."
In addition, the State Department is putting in place visa bans on a number of officials and individuals responsible for or complicit in threatening the sovereignty and territorial integrity of Ukraine.
The order was announced as U.S. Secretary of State John Kerry began a meeting with Russian Foreign Minister Sergei Lavrov in Rome.
A senior State Department official said the United States had informed the Europeans beforehand about the sanctions.
Obama is attempting to rally global opinion against the Russian move, which Russian President Vladimir Putin says was aimed at protecting ethnic Russians in the Crimea region of southern Ukraine.
The United States wants Russian troops to return to their bases in Crimea and for Moscow to allow international monitors into the region to ensure the human rights of ethnic Russians there are protected.
"We call on Russia to take the opportunity before it to resolve this crisis through direct and immediate dialogue with the government of Ukraine," the White House said.
The Obama order targets any assets held in the United States by "individuals and entities" responsible for the Russian military intervention in Ukraine, threatening its territorial integrity or seeking to assert governmental authority over any part of Ukraine without authorization from the Ukrainian government in Kiev.