As tanks and troops sent by the central government in Kiev zeroed in to Ukraine's wayward town of Slovyansk on Thursday, Moscow responded by ordering snap military drills on the border with the troubled region.
The move marks the first time Russia has officially acknowledged that its military action was directly related to events unfolding in neighboring Ukraine. During earlier military drills, the Kremlin had denied any connection with the Ukraine crisis.
But on Tuesday, Defense Minister Sergei Shoigu said Russia had been "forced to react to these developments," adding that Ukraine had deployed 160 tanks, 230 armored vehicles and at least 150 guns and mortars to counter 2,000 militants armed with about 100 machine guns.
"The forces are not equal. They were allowed to use arms against civilians. If this military machine does not get stopped today, it will lead to more bloodshed," Shoigu said, Interfax reported.
Shoigu did not specify the amount of troops that would participate in the exercise, though he did specify that Russia's Air Force would conduct patrol flights around the Ukrainian border.
Earlier in the day, President Vladimir Putin accused the new Kiev government of committing "serious crimes against its own people" and sapping last week's deal aimed at de-escalation in conflict-ridden Ukraine.
"We have participated in the Geneva meeting, where we signed certain documents ordering people from both sides to disarm, vacate administrative buildings and so on. What is happening? The Right Sector, as well as other radical organizations do not disarm, on the contrary, these gangs are getting legalized," Putin said in St. Petersburg on Thursday, referring to the radical nationalist movement that was instrumental in bringing down the pro-Moscow regime in Ukraine in February.
Ukraine's army units started pouring into Slovyansk on early afternoon Thursday, launching an attack against militants at checkpoints that guard each entrance to the town of 129,000. In the past two weeks Slovyansk has been the epicenter of pro-Russian separatist zeal in the Donetsk region of Ukraine, which has made it a target for Ukraine's authorities.
Ukraine's Acting President Oleksander Turchynov on Thursday demanded that Russia stop interfering in his country's domestic affairs.
No official information about the number of troops deployed in Slovyansk was released publicly by the Ukrainian officials by Thursday evening.
Speaking at the media forum in St. Petersburg on Thursday, Putin said that a crackdown against civilians by the regular Ukrainian Army would destroy whatever legitimacy the authorities in Kiev might have had before.
"If the regime in Kiev has done it then we cannot call them anything but junta or simply some kind of clique. They do not have a nation-wide mandate. They have got, at best, only elements of legitimacy in the framework of parliament. All other agencies cannot be called legitimate for one reason or another," Putin said, addressing a crowd of regional journalists.
The clash with pro-Russia militants led to at least five casualties among the separatists, according to Ukraine's security service. The operation was carried out not by the officers from the local branch but by the commandos dispatched from other regions. "At least three checkpoints" have been destroyed, the service said on its website Thursday. The town itself remained under control of the militants.
The violence has flared up despite agreements reached in Geneva last week, where Russia, Ukraine, U.S. and EU inked first written document over the crisis, committing themselves to refrain from the use of force and disarming all militant groups. Since then all sides have accused each other of breaching its stipulations.
At the same time, the militants that occupied multiple government buildings in the Donetsk region failed to vacate them, while civilian combatant groups on both sides of the conflict ignored the call to disarm.
Leader of the self-proclaimed secessionist Donetsk People's Republic, Denis Pushilin, that claims authority over the area, said earlier Thursday that they will not participate in the upcoming May 25 presidential election in Ukraine and will conduct a referendum over their status on May 11.