“The proposed referendum on the future of Crimea would violate the Ukrainian constitution and violate international law,” Obama said in a brief statement from the White House. “Any discussion about the future of Ukraine must include the legitimate government of Ukraine.
“In 2014, we are well beyond the days when borders can be redrawn over the heads of democratic leaders,” he added.
Reports earlier Thursday said that the pro-Moscow government in Crimea intended to hold a referendum on joining Russia.
Russian military forces in bases in that area of Ukraine last weekend seized control of the region. Russian President Vladimir Putin says that he is protecting the safety of ethnic Russians in Crimea, but the administration has urged him to pull back forces and allow international observers, characterizing the intervention as a power grab.
Obama has called the military action a violation of international law and warned that there would be “costs” for Moscow.
But despite those threats, Putin has ignored calls to stand down, insisting both that Moscow has not seized control of the region and that ethnic Russians there are in danger — both claims rejected by Washington.
In recent days, the administration has sought to raise pressure on Russia, cancelling trade and military talks and offering a $1 billion aid package to help Ukraine stabilize its economy.
The International Monetary Fund is also working with Kiev on assistance and the European Union has offered a two-year $15 billion package.
Earlier Thursday, Obama also signed an executive action allowing for targeted sanctions against individuals that the administration finds “responsible for destabilizing Ukraine.” The State Department is also authorized to ban visas for those individuals.
“These decisions continue our efforts to impose a cost on Russia and those responsible for the situation in Crimea,” said Obama. “And they also give us the flexibility to adjust our response going forward based on Russia's actions.”
The EU on Thursday followed the U.S. and announced some sanctions on specific individuals, including former Ukrainian President Viktor Yanukovych, who directed a brutal crackdown against opposition forces.
Obama said the international community would “remain firm” against Russia’s military incursion.
“I've spoken to several of our closest friends around the world and I'm pleased that our international unity is on display at this important moment,” said Obama.
The president though added that there was still time to resolve the crisis, urging Moscow to sit down for talks with leaders in Kiev, allow international observers to protect civilians in Ukraine and pull back forces to Russian bases.
“That's the path of de-escalation,” said Obama. “And Secretary Kerry is engaged in discussions with all of the relevant parties, including Russia and Ukraine, to pursue that path.”
Obama also called on lawmakers on Capitol Hill to “follow-up on these words with action” and pass legislation allowing the IMF to lend resources to Ukraine.
This story was published at 1:32 p.m. and has been updated.