WASHINGTON (AP) — Secretary of State John Kerry is warning Russia that it will face an immediate, “very serious series” of steps from the United States and Europe if it annexes Ukraine’s strategic Crimea region.

Speaking Thursday after German Chancellor Angela Merkel said Russia risks “massive” political and economic consequences if it refuses to soften its position, Kerry told a Senate committee that Moscow should expect the U.S. and European Union to take measures against it on Monday should it accept and act on the expected results of this weekend’s referendum in Crimea.