Ukrainian units backed by armored personnel carriers blocked all approaches to the town of Slovyansk, Russia’s state- run RIA Novosti news service reported, citing an unidentified pro-Russian activist. Two militants were wounded when an airport in Kramatorsk was stormed, forcing the protesters to retreat, according to RIA.

The government in Kiev started the operation after fighting between its forces and pro-Russian separatists turned deadly this week. The U.S. and the European Union also deliberated deepening sanctions against Russia, which they blame for stoking the unrest, as Barack Obama and Russian President Vladimir Putin remained at odds over who was at fault.

“The aim of these actions is to protect people,” acting President Oleksandr Turchynov said in a statement on his website. “Apart from Russian special forces and terrorists, there’s hundreds of thousands of innocent Ukrainian people deceived by Russian propaganda, and that is why we will take any needed anti-terrorist actions prudently and responsibly.”

The U.S. and EU states say Russia is behind the turmoil that has fueled their worst standoff since the Cold War. Russia, which NATO says has 40,000 troops massed on Ukraine’s border, denies involvement and says the government in Kiev isn’t protecting Russian-speaking citizens. In warnings similar to those that preceded its annexation of Crimea last month, it insists it has the right to protect them with force if needed.

At least one Ukrainian serviceman died over the weekend after a gun battle with armed separatists in the town of Slovyansk. Turchynov accused Russia of sending special forces to seize buildings in the country’s east, where activists occupied police stations in the cities of Horlivka, Kramatorsk, Donetsk and Mariupol.

“Blood was spilled once again in Ukraine,” Russian Prime Minister Dmitry Medvedev said on Facebook today. “There’s a sense in the country that a civil war could break out.”

Putin “is getting many requests” from eastern Ukraine “to intervene in one way or another,” his spokesman, Dmitry Peskov, told reporters yesterday.

In a phone call overnight, Obama “expressed grave concern about Russian government support for the actions of armed, pro- Russian separatists” and urged Putin to get them to withdraw from facilities they seized, according to a White House statement. Putin told Obama he should “use the American side’s capabilities to prevent the use of force and bloodshed as much as possible,” according to a Kremlin statement.