White House press secretary Jay Carney on Monday said that the U.S. would not view a referendum in Crimea to leave Ukraine and join Russia as “legitimate.”

Asked if the U.S. would be able to prevent the pro-Russian government in Crimea from holding the vote, Carney told reporters that he “can't predict the future.” But he insisted that the referendum “won't be viewed by the United States as legitimate.”

“It's inconsistent with the Ukrainian constitution, which says that any changes to Ukraine’s borders have to be decided by all of Ukraine,” said Carney. “Other countries have made it clear it will be viewed as illegitimate as well.”

Carney declined to say what measures the U.S. would take if Russia allows Crimea to go ahead with the vote, but cited sanctions Obama authorized last week that would allow the U.S. to target individuals found to be undermining democracy in Ukraine and the country's territorial integrity.

The U.S. though has yet to name any individuals to be sanctioned.

Carney said the measures established by Obama were “flexible” and would “allow for action to be taken that holds people accountable.”

President Obama has called on Moscow to de-escalate the situation in Crimea, after Russian troops seized control of the Ukrainian region.

Russian President Vladimir Putin has ignored calls to pull back his troops, sit down with the interim government in Kiev and allow international monitors.

Obama has called the military incursion a violation of international law and has urged Putin to block Crimea from holding the referendum on seceding from Ukraine.

Putin says that Moscow is protecting the rights of the majority ethnic Russian population in the region, but the White House has dismissed those claims, painting the incursion as a power grab.