Kerry meets Lavrov in London over the crisis in Ukraine

US Secretary of State John Kerry has met with his Russian counterpart Sergey Lavrov in London for talks over the situation in Ukraine prior to a planned referendum in Crimea.

The meeting took place at the home of the US ambassador in London and came just one day before the planned referendum that will ask Crimeans’ view on whether the region should break away from Ukraine and join Russia.

The US is seeking to head off the vote and has threatened Russia with sanctions and “additional costs” if Moscow recognizes the results of the planned referendum.

“I look forward to the opportunity to dig into the issues and possibilities that we may be able to find on how to move forward together to resolve some of the differences between us,” Kerry told reporters before his meeting with Lavrov.

On Thursday, Kerry issued an ultimatum to Russia, saying Moscow has until Monday to “reverse course” over Ukraine if the last-ditch diplomatic talks in London fail to resolve the differences between the two countries.

“There will be a response of some kind to the referendum itself,” Kerry said before a Senate hearing. “If there is no sign [from Russia] of any capacity to respond to this issue … there will be a very serious series of steps on Monday.”

Later in the day, White House press secretary Jay Carney also issued new threats against Russia, saying Moscow could face “sanctions” and “additional costs” if it “continues down the path that it is currently on” over the crisis in Ukraine.

On Tuesday, Carney also said that Washington’s military exercises with Romanian and Bulgarian warships in the Black Sea “speak in a clear voice” to Russia.

A US Navy destroyer is set to take part in joint military drills in the Black Sea while in Poland, US fighter jets and hundreds of American troops are planned to participate in military exercises.

This comes as Russia has gathered thousands of its troops near its border with Ukraine to conduct military exercises.