Pro-Russian separatists are occupying government buildings, and protesters are issuing calls for referendums. Meanwhile, Moscow watches intently, as a steady influx of troops amass just across the border.

This isn't Crimea; it's the new eastern Ukraine.

U.S. Secretary of State John Kerry on Tuesday accused Russia of "an illegal and illegitimate effort" to destabilize Ukraine, after days of protests swept the region.

"Russia’s clear and unmistakable involvement [...] is more than deeply disturbing," Kerry said. "No one should be fooled, and believe me, no one is fooled by what could potentially be a contrived pretext for military intervention just as we saw in Crimea."

Kerry compared the unrest in three eastern Ukrainian cities — Donetsk, Luhansk and Kharkiv — to the preceding actions that led to a Russian invasion and eventual annexation of Ukraine's Crimea region in the south.

"It is clear that Russian special forces and agents have been the catalyst behind the chaos of the last 24 hours," Kerry said.

"The United States and our allies will not hesitate to use 21st-century tools to hold Russia accountable for 19th-century behavior."

Over the past few days, pro-Russian protesters have stormed government buildings, clashed with police and called for each city's independence from Ukraine.

In Donetsk, several hundred activists broke into the Regional Administration Building and barricaded themselves inside. In Luhansk, the BBC says police battled dozens of protesters who stormed the local security service building in an attempt to free pro-Russia activists who had been arrested earlier in the week. And in Kharkiv, several were injured after dozens of protesters were expelled from the regional state administration building on Tuesday.

"I call this a theater of the absurd," Donetsk Governor Serhiy Taruta said of the events. "It is just artists performing, but the main thing is that there is an ever-dwindling audience."

Image: Olga Ivashchenko/Associated Press

Ukraine's acting Interior Minister Arsen Avakov, who was in Donetsk to oversee the operation, described it on his Facebook page as an "anti-terrorist operation" that occurred "without firing a shot, grenades or other special weapons."

Avakov said the operation was a part of a "broader redeployment in the region" meant to fight back against the unrest that Russia has been accused — by both Kerry and Ukraine — of orchestrating, according to The New York Times.

However, the activists at government building in Donetsk say "not true," BuzzFeed's Mike Giglio, who spent the night in the building, reports. “Tell them we are not paid!” one 24-year-old reportedly said. “And they said we were Russian spies — it isn’t true. The only Russians here are the ones from the TV channels.”

Image: Efrem Lukatsky/Associated Press

In response, the Russian Foreign Ministry warned that any use of force from the Ukrainian military could lead to terrible consequences for the country. "We are calling for the immediate cessation of any military preparations, which could lead to civil war," the ministry said in a statement on its website, according to CNN.

Russia has been building up its troops just across the border from the three eastern Ukrainian cities, and refused to say why.

The buildup in Russia's Rostov and Belgorod regions is substantial, said U.S. Ambassador Daniel B. Baer, the U.S representative to the Organization for Security and Cooperation in Europe (OSCE). He also urged Russia to reduce its troops to pre-crisis numbers and positions.

"The scope of the forces includes not only ground forces, but also forward-deployed air power, providing quick-strike capability and air support to potential ground operations," he said in a statement at the Permanent Council, in Vienna on April 7.

NATO Secretary-General Anders Fogh Rasmussen warned last week that any Russia intervention further in Ukraine would be a "historic mistake" that would "lead to further international isolation of Russia."

Meanwhile, the U.S. sent a second Naval warship to the Black Sea in a sign of support for Ukraine — a move Russia immediately criticized. NATO will also triple its number of fighter jets patrolling the region.

Video of Kerry's remarks on escalating tension in eastern Ukraine