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The United Nations Security Council will meet in an emergency session Sunday evening to discuss the crisis in Ukraine, senior U.S. officials told NBC News.

The meeting is expected to take place at 8 p.m. ET at the United Nations headquarters in New York City.

The Ukrainian crisis escalated on Saturday when pro-Russian militants took control of government buildings in Slovyansk, eastern Ukraine.

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In response, Ukraine launched what was termed an “anti-terrorist operation” in that city, urging residents to stay indoors in the expectation of clashes between pro-Russian separatists and government security forces.

Military helicopters were seen outside the besieged buildings on Sunday, and each side had reported deaths, but details of reported clashes were unclear.

Acting Ukraine President Oleksandr Turchynov appeared on state television late Sunday and offered amnesty to any rebels occupying state buildings who would lay down their weapons by Monday morning.

Turchynov said the Ukrainian Security Council decided to deploy the army because "we're not going to allow Russia to repeat the Crimean scenario in Ukraine's east."

Meanwhile, former Ukrainian President Viktor Yanukovych appeared on Russian television and called for an immediate referendum to “stop the civil war that has begun.”

Yanukovych, who was ousted after bloody Kiev protests, said the American CIA was behind a decision by Ukraine to take military action. He claimed CIA chief John Brennan had met with Ukraine’s new leadership and "in fact sanctioned the use of weapons and provoked bloodshed."

In a statement, the CIA refuted Yanukovych's assertions.

"We have no comment on Director Brennan's travel itinerary as we do not speak publicly about the CIA Director's travel, but the claim that Director Brennan encouraged Ukrainian authorities to conduct tactical operations inside Ukraine is completely false," CIA spokesman Todd Ebitz said, adding that like other U.S. officials, Brennan strongly believes in a diplomatic resolution to the crisis.

Jeff Black and Robert Windrem of NBC News as well as Reuters contributed to this report.