The White House blamed “increased Russian pressure” for new unrest in Ukraine and urged the Kremlin to “cease all efforts to destabilize” their neighbor.

“We're concerned about several escalatory moves in Ukraine over the weekend, and we see those as a result of increased Russian pressure on Ukraine,” White House press secretary Jay Carney said Monday.

Carney's comments came after pro-Russian protesters seized government buildings in the city of Donetsk in Ukraine and declared their own republic, according to reports. The protesters called on Moscow to send in troops to protect them from Kiev and demanded a referendum on secession from Ukraine, similar to the vote held in Crimea.

Carney dismissed those calls, saying that there was “strong evidence” that many of these “demonstrators were paid and not local residents.”

“These people lack the legal authority to make any of those decisions,” Carney said about the pro-Russian groups’ demands.

The seizure of the government buildings came after reports that a Ukrainian military officer was killed after a confrontation with Russian troops in Crimea, further raising tensions.

Russia's annexation of Ukraine's Crimea province sparked an international crisis and presented President Obama with his toughest diplomatic test. But the U.S. and allies have been unable to force Russian President Vladimir Putin to back down.

In recent weeks, Russian troops have massed along the border with Ukraine. Moscow says it is not planning an invasion but has vowed to protect the interests of ethnic Russians in Ukraine.

The U.S. has urged Russia to move troops back from the border region and begin talks with the interim government in Kiev.

Carney again warned that the U.S. was ready to step up sanctions imposed by the U.S. and European Union on Russian and Ukrainian officials if Moscow made further moves to violate Ukraine's sovereignty.

“If Russia moves into Eastern Ukraine, either overtly or covertly, this would be a very serious escalation,” he said. “We call on President Putin and his government to cease all efforts to destabilize Ukraine, and we caution against further military intervention.”

President Obama has laid the groundwork for sanctions targeting key sectors of the Russian economy, but there is little consensus on tougher measures, with European leaders fearing economic blowback because of their energy dependence on Moscow.

“We've made very clear that should Russia take action that violates Ukraine's territorial integrity further or violates Ukraine's sovereignty further, there will be further consequences,” said Carney.