Tensions flared over the weekend as Russian officials vowed to respond to the Ukrainian shelling of a Russian border town, amid the offensive by troops of the Western-backed Ukrainian regime in Kiev against pro-Russian militias defending the eastern Ukrainian cities of Donetsk and Luhansk.

On Sunday, shells hit the Russian town of Donetsk, which has the same name as the nearby Ukrainian city of Donetsk that is a center of opposition to the Kiev regime. The shells killed a 46-year-old father of four and wounded four, including two elderly women.

The shelling came after repeated attacks across the Russian border by Kiev regime forces. On Saturday, Ukrainian forces fired several shells at Russia’s neighboring Kuibyshev district and started a gunfight with Russian border guards there. On Sunday, Ukrainian warplanes violated Russian air space and overflew Donetsk, Russia shortly after the shelling had taken place, before launching missiles at targets inside Ukraine.

The Russian Foreign Ministry summoned the Ukrainian chargé d’affaires in Moscow to formally protest what it considered to be an act of aggression against sovereign Russian territory. Its statement warned Kiev of possible Russian retaliation: “This incident is evidence of the very dangerous escalation of tension in the Russian-Ukraine border area, and could have irreversible consequences, the responsibility for which lies with the Ukrainian side.”

Russian Deputy Foreign Minister Grigory Karasin said there would be a “rigorous and concrete answer” to the Ukrainian shelling: “Naturally, this action will not be left without a corresponding reaction. The talk with the Ukrainian side on this issue is going to be serious and tough.”

The Kiev regime’s security officials denied responsibility for the shelling, blaming it on pro-Russian opposition militias. “Ukrainian troops are definitely not shelling the territory of the Russian Federation. We did not shoot,” said National Security Council of Ukraine (NSCU) spokesman Andrey Lysenko.

Officials of the Donetsk People’s Republic (DPR) blamed the shelling on Kiev regime forces, however. “We are accustomed to being blamed for all Ukrainian shellings,” DPR Vice-Prime Minister Andrey Purgin told Russia’s Moscow Speaks radio station.

In recent weeks, Kiev regime forces have repeatedly attacked Russian positions, including three key border posts through which Ukrainian refugees fleeing the fighting in Donetsk and Luhansk are trying to cross into Russia. In addition to this weekend’s shelling of Donetsk, Russia, mortar fire hit the Novoshakhtinsk checkpoint on June 20 and the Gukovo checkpoint on June 28. The Novoshakhtinsk checkpoint was again shelled on July 3 and July 5. Until this weekend, however, there had been no fatalities from Ukrainian fire in Russia.

After the latest shelling, Russian officials announced they would move camps built to house the influx of Ukrainians refugees further into Russia, away from the Ukrainian border. At least 22,000 Ukrainian refugees are currently living in temporary shelters in Russia, and DPR officials warned last week that “hundreds of thousands” of east Ukrainians could ultimately flee the battle zone.

Ukrainian attacks on Russian soil expose the utterly reckless character of the far-right regime that emerged from the Western-backed, fascist-led putsch this February in Kiev, and the lies promoted by Western governments and media that Russia is the aggressor in the conflict. Ukrainian forces, working closely with Washington and European Union officials to crush opposition in east Ukraine, are threatening to provoke a war with Russia.

As they mobilize tanks, fighter jets, and attack helicopters to assault east Ukrainian cities and bomb Russia, Kiev regime officials are threatening a bloodbath. Last week, Ukrainian President Petro Poroshenko vowed disproportionate retaliation for his forces’ losses: “For every soldier’s life, the militants will pay with dozens and hundreds of their own.”

As part of its efforts to maintain good relations with the Western imperialist powers, Russia has repeatedly indicated that its current policy is not to intervene militarily to attack the Ukrainian army and crush its offensive in eastern Ukraine. However, an anonymous source close to the Kremlin told the Guardian that a Russian military intervention to defend ethnic Russians in east Ukraine is still possible. “I think all it would take would be one day where, say, 300 people are killed in the east and Putin will be simply obliged to act. I don’t think you can rule it out yet, not at all,” the source said.

Such an intervention would threaten to unleash an all-out war between Russia and Ukraine, in which NATO could intervene to prevent an overwhelming Ukrainian defeat—directly posing the risk of war between nuclear-armed powers.

The bloodbath threatened by the Kiev regime is an indictment of the far-right forces and affluent middle class pseudo-left groups that Washington, Berlin, and their allies mobilized in the pro-European Union (EU) protests on Kiev’s Independence Square (Maidan), leading to the February putsch that toppled pro-Russian Ukrainian President Viktor Yanukovych.

While Ukrainian army units initially refused to attack the east Ukraine opposition forces, the imperialist powers have worked closely with the Kiev regime to reshape the Ukrainian army into a force capable of waging war against its own people. Over past months, fascist militias like Right Sector that led the Maidan protests and private armies funded by Ukrainian billionaire oligarchs have been merged with the Kiev regime’s National Guard.

“They have overcome that psychological barrier in which the military were afraid to shoot living people…not simply to shoot living people, but their own people. After the forces were restructured a bit, and it became clear who were our people, who were foes, the operations became more effective,”

Mykola Sungurovskyi of Kiev’s Razumkov Center think tank told the New York Timeslast week.

The Times also spoke with Kiev regime security official Andriy Parubiy, a founding member of the fascist Social-National Party of Ukraine, who made clear that Kiev sees this offensive as a model for other former republics of the Soviet Union.

“There are many other countries which are not ready—properly speaking, their armed forces are not ready, are unprepared, for this type of war. We, of course, studied the experience of both Croatia and Israel, but here a lot of new features are added. And, if Russia sees this experience is successful, this experience can very easily be used in any Baltic countries, and even in Belarus and Kazakhstan,”

Parubiy told the Times .

The Kiev regime is working closely with US and EU officials as they seek to undermine Russian influence in Ukraine and throughout the region. Last month, US State Department official Gregory Kausner traveled to Ukraine to discuss financial and operational assistance to the National Guard, according to the Ukrainian Interior Ministry’s web site.

German officials have also given their support to the actions, while issuing a few cynical statements in an attempt to distance themselves from the worst of the bloodshed. German government spokesman Steffen Seibert reported that in a phone call last Thursday with Poroshenko, German Chancellor Angela Merkel “urged President Poroshenko to maintain a sense of proportion in his legitimate actions against the separatists and to protect the civilian population.”

The result of this reactionary offensive, which has no support in the working class in either the Western imperialist powers or the former Soviet republics, is a disaster for the Ukrainian people. Train and bus tickets out of Donetsk and Luhansk are all booked, as hundreds of thousands of residents take public transport or their own vehicles to flee the cities, fearing a long-term siege by Kiev regime forces.

DPR officials began an official evacuation of residents of the industrial Donetsk suburb of Maryinka on Saturday, as Ukrainian forces’ shelling of the suburb killed thirty people.

They also reported that they were recruiting large numbers of volunteers to fight the Kiev regime’s forces. “We are not holding a forced mobilization. We are counting on volunteers,” DPR mobilization department chief Pavel Gubarev told Russia’s ITAR-TASS news agency. “Over the past week, over a thousand volunteers have enlisted in our army.”