Ukrainian Prime Minister Arseniy Yatsenyuk accused Russia on Friday of wanting to start a third world war.
“The world has not yet forgotten World War II, but Russia already wants to start World War III," Yatsenyuk told his interim cabinet, according to The Guardian.
He warned Russia’s interference in Ukraine could spread to wider conflict.
"Attempts at military conflict in Ukraine will lead to a military conflict in Europe,” he added.
The United States has already deployed troops to Baltic nations for military exercises, and Russia announced new exercises along Ukraine's border on Thursday.
The prime minister’s remarks were some of his strongest since the crisis escalated last month.
President Obama, meanwhile, said at a press conference in South Korea that he would consult “key European leaders” about the crisis in Ukraine on Friday and discuss the possibility of moving forward with tougher sanctions against Russia.
Lawmakers have been calling on the Obama administration to step up U.S. sanctions because the previous rounds have not yet deterred Russian President Vladimir Putin.
Secretary of State John Kerry warned Russia at an unscheduled press conference Thursday evening that its window to change course in Ukraine is closing.
"Let me be clear," Kerry said. "If Russia continues in this direction, it will not just be a grave mistake, it will be an expensive mistake."
Kerry asserted that Russia is guilty of continuing “to fund, coordinate and fuel” the pro-Russian separatist movement in Ukraine.
Last week, diplomats from Ukraine, Russia, the U.S. and the European Union reached a deal to ease tensions in Ukraine. Among its components, the agreement said separatists must leave the government buildings they’ve been occupying in Ukraine, and that they will receive amnesty if they haven’t committed crimes. Many separatist groups have rejected the deal.
“Let's get real," Kerry said Thursday. "The Geneva agreement is not open to interpretation."