The White House says it has strong evidence that some pro-Russian protesters who took over government buildings in three eastern Ukrainian cities on Sunday were not local residents, and were paid for their actions.
A White House spokesman told reporters Monday that the Obama administration is telling Russia to stop intervening in eastern Ukraine, and is threatening more sanctions if Moscow does not comply.
He said if Russia moves into eastern Ukraine either overtly or covertly, it would be considered "a very serious escalation."
U.S. Secretary of State John Kerry spoke with his Russian counterpart, Sergei Lavrov, on Monday, in the latest of a long series of discussions about Ukraine. A State Department spokeswoman said Kerry told Lavrov that Sunday's events in eastern Ukraine did not appear to be "spontaneous." Kerry called on Russia to "publicly disavow the activities of separatists, saboteurs and provocateurs."
The Pentagon confirmed that a U.S. warship is headed to the Black Sea to provide support to European allies in the region. The ship is expected to arrive within a week.
NATO announced Monday that it is limiting Russian diplomats' access to NATO headquarters in Brussels to the Russian ambassador, his deputy and two support staffers, because of the conflict over Crimea.
Earlier, Ukraine's acting prime minister accused Russia of backing the protests in the cities of Donetsk, Luhansk and Kharkiv.
Arseniy Yatsenyuk told a Cabinet meeting Monday that Russia is behind the unrest. He said the demonstrations are clearly part of a plan to destabilize the situation and allow "foreign" troops to cross the border and seize Ukrainian territory.
Ukraine's acting president, Oleksandr Turchnyov, also blamed Russia for Sunday's demonstrations and accused it of "playing out the Crimean scenario." He vowed anti-terrorist measures will be carried out.
Russia's foreign ministry responded that Ukraine should stop blaming Russia for its problems. Moscow repeated its call for constitutional reform and allocation of more power to Ukraine's regional governments.
Protesters in Donetsk who have seized control of the government's main administration building announced the creation of a sovereign "people's republic" independent of Kyiv. The demonstrators have erected a barricade of tires and barbed wire to keep security forces from retaking the structure. The protesters said a referendum on creating an independent "Donetsk people's republic" would be held no later than May 11.
An unidentified protester inside the government building in Donetsk posted a video online asking Russian President Vladimir Putin to send "peacekeeping troops" to the region.
In Luhansk, activists took control of facilities where weapons are stored, and in Kharkiv, security forces briefly reclaimed an occupied building before the protesters took it back.
The protesters in Donetsk, Luhansk, and Kharkiv demanded the cities hold the same kind of referendum held last month in Crimea, which called for independence from Ukraine with an eye toward Russian Federation membership.
Opposition presidential candidate Yulia Tymoshenko visited Donetsk on Monday and said she believes the separatist movement there was instigated by the Kremlin. She said the referendum the Donetsk protesters demand will not be carried out.
Ukrainians have been split between those loyal to the Kyiv government and those calling for stronger ties to Russia, following Russia's annexation of the Crimean peninsula last month.