Russia's President Vladimir Putin looks on during a meeting in his Novo-Ogaryovo residence outside Moscow, on April 7, 2014.
The United States told Russian President Vladimir Putin on Monday to stop destabilizing Ukraine, warning it had evidence that clashes involving pro-Kremlin forces were incited by outsiders and raised the threat of new sanctions.
As tensions again spiked between Moscow and Kiev, Washington also dispatched a destroyer to the Black Sea in a show of gunboat diplomacy, but also tried to organize talks on the crisis with Russia, Ukraine and the European Union.
Washington's sharp warning followed weekend incidents in Ukraine, in which thousands of activists chanting "Russia!" seized government buildings in the cities of Kharkiv and Donetsk and a security service headquarters in the eastern region of Lugansk.
"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," said White House spokesman Jay Carney.
"There is strong evidence suggesting some of these demonstrators were paid and were not local residents," said Carney, who would not be drawn on whether the White House believed Russia was using covert operatives to stir up trouble to justify military action to protect ethnic Russians in Ukraine.
He did warn, however, that attempts by Russia to move into eastern Ukraine after its annexation of Crimea, either "overt or covert" could trigger increased sanctions against Moscow's economy threatened by Washington last month.
The measures would augment already imposed punishments against prominent politicians in Russia and what officials described as a "crony" bank, by directly targeting the Russian economy in the financial services, energy, metals and mining, engineering and defense materials sectors.
Such sanctions could provide a test for European nations which are much more entwined with the Russian economy than the United States, and would have to consider to follow suit.
"It would not be our preferred course because there would be harm done to the US economy, to the global economy, to the European economy," said Carney.
"But it would be necessary to do so if Russia engaged in further transgressions against Ukraine."
In a new show of support for allies in the region, sources said that the Aegis Ballistic Missile Defense System destroyer USS Donald Cook was being sent to the Black Sea.
"The purpose is primarily to reassure our allies and partners in the region that we're committed to the region," said Pentagon spokesman colonel Steven Warren.
Warren did not disclose the name of the vessel or its mission, but sources confirmed it was the Cook, which has been based at Rota naval base in Spain.
The State Department echoed White House warnings but also disclosed that Secretary of State John Kerry spoke to Russian Foreign Minister Sergei Lavrov about convening talks within 10 days to de-escalate tensions involving the European Union, Russia, Ukraine and the United States.
"The details and the agenda will be worked out in the coming days," said State Department spokeswoman Jen Psaki.
Political pressure on Kiev's embattled leaders hit new heights on Sunday, when thousands of pro-Russia activists seized the buildings in Kharkiv and Donetsk and security service headquarters in Lugansk.
The Donetsk activists went one step further by proclaiming the creation of a sovereign "people's republic" in the region of about five million people.
Ukraine's acting president Oleksandr Turchynov accused Russian special forces of inciting the uprising and Prime Minister Arseniy Yatsenyuk warned Russia was trying to "dismember" Ukraine.
The rising tensions came a week after President Barack Obama returned from a trip to Europe he used to solidify the Western alliance and to try to persuade European leaders to embrace strong action if Russia goes further into Ukraine.