The UN Security Council has held an emergency meeting on the escalating crisis in eastern Ukraine.
The meeting was called for by Russia.
It came after Ukraine’s acting President Oleksandr Turchynov gave pro-Russian separatists until April 14 at 0600 GMT (0800 Prague time) to hand in their weapons and leave government buildings they have occupied in several eastern cities, or face a "large-scale anti-terrorist operation" involving the army.
Russian Ambassador Vitaly Churkin told the Council he was alarmed by Ukraine's announcement.
Churkin said, "Kyiv should stop the use of force against eastern Ukrainian people and start a genuine dialogue."
But U.S. Ambassador to UN Samantha Power said instability in Ukraine is "completely man made," and "it was written and choreographed in and by Russia."
Powers reaffirmed that "United States stands with Ukraine and the fundamental principle that the future of Ukraine must be decided by the Ukrainian people."
Yuriy Sergeyev, the Ukrainian ambassador to the UN, also said the crisis was artificially created by Russia, and called on Moscow to "stop immediately actions aimed at undermining Ukraine's territorial integrity."
Sergeyev said that based on intelligence gathered by Ukrainian intelligence over the last month and a half, "in the eastern regions of Ukraine, agents of the Russian special service embarked on a large-scale operation."
Sergeyev said the aim of theoperation was "to seize power, destabilize the situation, threatening the lives of citizens of Ukraine for the separation of those regions from our country."
Earlier on April 13, speaking in a live televised address from parliament, Turchynov accused Russia of waging "war" against Ukraine by sowing disorder in the country's east.
Turchynov said authorities in Kyiv will not allow the "repetition of the Crimean scenario."
Russia annexed the Crimean Peninsula in March after local residents voted to secede from Ukraine and seek to join Russia in a referendum.
In a decree, Turchynov offered not to prosecute any militants who gave up their weapons by 9 a.m. (0600 GMT/UTC) on April 14.
Russia's Foreign Ministry said Kyiv's decision to use the armed forces against protesters was "outrageous," and demanded that the authorities "immediately stop waging war again their own people."
"It is now the West's responsibility to prevent civil war in Ukraine," it said in a statement.
WATCH: Ukrainian government troops and helicopters arrived outside Slovyansk after the city's police station was overrun by gunmen. (Reuters/Espresso TV)
Turchynov's announcement came after pro-Russia militants took over Slovyansk on April 12 and targeted several other municipalities in eastern Ukraine.
The activists are demanding a referendum on autonomy and possibly joining Russia.
Interior Minister Arsen Avakov said one Ukrainian security officer was killed, and five others wounded in a gunfight with pro-Russia activists in Slovyansk on April 13.
He said both sides suffered a number of casualties.
Reports said armed men also took over police stations and official buildings in Kramatorsk, Krasny Liman, Mariupol, and other cities.
In Donetsk, the pro-Russia activists continue to occupy the main government building that they seized a week ago.
Meanwhile, pro-Russia protesters and pro-Ukraine demonstrators clashed in the eastern city of Kharkiv. Interfax news agency quoted Kharkiv authorities as saying 10 people were injured in the violence.
Kyiv and the West accuse Russia of intimate involvement in the trouble -- a charge denied by Moscow.
With reporting by AP, Reuters, AFP, and Interfax