"If the referendum takes place, there will be some sanctions; there will be some response, put it that way," Kerry told reporters after the last-ditch, six-hour talks with Russian Foreign Minister Sergei Lavrov.

"If Russia does establish facts on the ground that increase tensions or that threaten the Ukrainian people, then obviously that will beg an even greater response, and there will be costs."

Kerry said that while Russian President Vladimir Putin has pledged to respect Sunday's vote in the Black Sea peninsula, that offers him "many options as to how he chooses to respect the vote.”

"If the people of Crimea vote overwhelmingly, as one suspects they will, to affiliate or be associated with Russia, you can respect the vote by making sure that their autonomy is increased, that their needs that have prompted that vote are properly respected -- without necessarily having to make a decision to annex," he said.

Kerry said the United States was prepared to respect Putin's interests and rights with regards to Crimea -- which has a large ethnic Russian population -- but that the referendum would be contrary to the Ukrainian constitution.

"He can actually have a claim to have served his purpose for protecting the people that he is interested in protecting by augmenting their rights and by asserting his prerogatives at the end of this effort," Kerry said.

"There are other options and that's what we're continually trying to say."

Hours of talks between Kerry and Russian Foreign Minister Sergei Lavrov in London on Friday on the crisis in Ukraine ended with no common vision.

NJF/NJF