Russia is building up its troop presence on the border with Ukraine.
Russian forces in the area now total 30,000 troops - up from 20,000 from Monday. President Obama warned Russia's president Vladimir Putin on Tuesday that he would face "additional costs" if Russia does move further into Ukraine.
CNN's Erin Burnett spoke with Former President Jimmy Carter and asked him whether there was a way to stop Russia at this point, short of war.
"I think we have to be very bold and strong in preventing Putin from having a temptation to move militarily into Eastern Ukraine," Carter said.