Secretary of State John Kerry has issued an ultimatum to Russia today, insisting they have until Monday to “reverse course” and abandon Crimea. The Crimean peninsula is holding a referendum on Sunday, in which it is expected to vote to join the Russian Federation.

Kerry said if Russia refuses to give in to US demands they could expect a “very serious series of steps” against them, including major economic sanctions, which US officials say could do serious harm to Russia’s economy.

The Obama Administration has been trying to coax significant Russian trading partners, of which the US is not, into agreeing to impose major sanctions that would harm their trade.

The US demands include Russia forcing Crimea to cancel the referendum and withdrawing all of its troops from the peninsula. Under treaty with the Ukraine, Russia is allowed to keep up to 25,000 troops in the Crimea.