Acting Ukrainian President Oleksandr Turchynov told the country in televised remarks that the seizure of government buildings in eastern Ukraine and the declaration of a "Donetsk People's Republic" were engineered by Russia.
Turchynov said that the actions that began the previous day in the eastern Ukrainian cities of Luhansk, Donetsk, and Kharkiv are "the second wave in Russia's special operation against Ukraine."
Moscow responded by accusing Kyiv of "finger-pointing" and suggested that "the political forces that call themselves the Ukrainian government...listen to legitimate demands."
U.S. Secretary of State John Kerry reportedly told Russian Foreign Minister Sergei Lavrov via telephone that events in eastern Ukraine did not appear to be "spontaneous."
Reuters quoted State Department spokeswoman Jen Psaki as saying the two discussed convening direct talks in the next 10 days between Ukraine, Russia, the United States, and the European Union to defuse tensions.
Pro-Russia activists occupying the regional government building in Donetsk earlier in the day on April 7 proclaimed the creation of a separatist republic.
A speaker inside the building called for a referendum on the region's future to be held by May 11 and asked for Russia to send a "peacekeeping contingent" to protect the separatists.
Pro-Russia activists also looted an armory in Luhansk.
Turchynov said those who had taken up arms against Ukraine "will be targeted in antiterrorist operations."
Donetsk Governor Serhiy Toruta urged the government to hold an urgent meeting about the national security in his region. He said in a statement that "a plan is being implemented in the Donetsk, Luhansk, and Kharkiv regions to destabilize peace as well as social and economic stability."
The Russian Foreign Ministry said Lavrov, in a telephone conversation on April 7, told acting Ukrainian Foreign Minister Andriy Deshchytsya that Kyiv must not use force against pro-Russian protesters in eastern Ukraine.
"If the irresponsible attitude toward the fate of the country, the fate of their own people, on behalf of the political forces that call themselves the Ukrainian government were to continue, Ukraine would inevitably face ever new difficulties and crises," Russian Foreign Ministry spokesman Aleksandr Lukashevich said. "Enough finger-pointing at Russia -- blaming it for all the troubles of today's Ukraine. The Ukrainian people need to hear from Kyiv clear answers to all questions. It is time to listen to these legitimate demands."
Earlier in the day, Ukrainian Prime Minister Arseniy Yatsenyuk accused Russia of being behind the building seizures.
Yatsenyuk said at a government meeting in Kyiv that "an anti-Ukrainian plan is being put into operation under which foreign troops will cross the border" and seize Ukrainian territory.
He said First Deputy Prime Minister Vitaliy Yarema would be in Donetsk to deal with the unrest.
Yatsenyuk said calls by pro-Russia protesters in eastern Ukraine for the federalization of the country are "aimed at dividing and destroying Ukraine."
"The huge protests that took place just a month ago have finished but a radical group of people -- 1,000-1,500 people -- with specific Russian accents stayed in the region," he said. "Coordinating their activities with special services of foreign states, they organized a riot and chose to capture public buildings and destabilize the situation."
Yatsenyuk added that Interior Minister Arseniy Avakov was in Kharkiv to deal with the situation there.
Pro-Russia protesters seized the local headquarters of the SBU, the Ukrainian security service, in Luhansk on April 6:
Yatsenyuk also noted that large numbers of Russian troops are still stationed along Ukraine's eastern border.
"[Russian] troops are located within a 30-kilometer zone from the Ukrainian border," he said. "In fact, no one withdrew any troops."
Yatsenyuk's comments came after pro-Russia activists who on April 6 had broken into the state security (SBU) headquarters in the city of Luhansk seized weapons from the SBU arsenal and set up barriers on a main street.
Police have since shut down all roads into Luhansk.
In Kharkiv, officials said the regional administrative building has been cleared of pro-Russia protesters who stormed the building on April 6.
Ukrainian Interior Minister Arsen Avakov said on Facebook that "separatist" protesters are no longer in the building, although some later threw firebombs and rocks at the windows of the building and set tires ablaze outside.
The regional government in Kharkiv said all offices are functioning normally.
At the city's broadcasting tower, however, pro-Russian protesters had taken control, demanding it resume broadcasting Russian television channels, according to Kharkiv Mayor Gennady Kernes.
But local police said the assailants later left.
Pro-Russia activists occupied the regional government buildings in Kharkiv and Donetsk along with the SBU headquarters in Luhansk.
Russian flags were hoisted on the buildings.
The Russian Foreign Ministry chided the government in Kyiv, which it does not recognize, suggesting it is acting irresponsibly.
"If the irresponsible attitude toward the fate of the country, the fate of their own people, on behalf of the political forces that call themselves the Ukrainian government were to continue, Ukraine would inevitably face ever new difficulties and crises," ministry spokesman Aleksandr Lukashevich said. "Enough finger-pointing at Russia -- blaming it for all the troubles of today's Ukraine. The Ukrainian people need to hear from Kyiv clear answers to all questions. It is time to listen to these legitimate demands."
In Washington, White House spokesman Jay Carney described the latest developments as the result of "increasing Russian pressure on Ukraine."
The White House also suggested that there was "strong evidence" that some of the pro-Moscow protesters "were paid and not local residents."
Separately, Ukraine's Defense Ministry said a Russian serviceman shot dead an unarmed Ukrainian naval officer in eastern Crimea.
The ministry said the killing took place on April 6 in the town of Novofyodorovka and that he was shot twice.
RFE/RL's Ukrainian Service said the officer was killed in a "campus room" where he had been staying ahead of his planned return to Ukraine with his family on April 9.
Ukrainian navy spokesman Vladislav Seleznyov said the the officer, Stanislav Karachevsky, was killed with an AK-47.
Russian forces seized Crimea last month and held an unrecognized referendum on the Ukrainian peninsula's status before Moscow annexed it as part of Russia in an internationally condemned action.