The standoff between Russia and the West over Ukraine is intensifying ahead of a referendum on secession in Ukraine’s Crimea region. Crimea residents are set to vote Sunday on whether to secede from Ukraine and join Russia. Secretary of State John Kerry is in London today for meetings with his Russian counterpart. On Thursday, Kerry warned Russia faces sanctions if the vote proceeds.

John Kerry: “There will be a response of some kind to the referendum itself. And in addition, if there is no sign of any capacity to be able to move forward and resolve this issue, there will be a very serious series of steps on Monday in Europe and here with respect to the options that are available to us. Now, our choice is not to be put in the position of having to do that. Our choice is to have a respect for the sovereignty and independence and integrity of the country of Ukraine.”

The United States is circulating a United Nations resolution to declare the referendum a violation of international law. But Russia has compared the secession of Crimea to Kosovo’s independence from Serbia, which the United States backed. Russia has begun massing troops along Ukraine’s eastern border.