Russia is threatening to use nuclear weapons if Ukrainian forces continue to fight pro-Russia separatists, the Ukrainian Defense Minister is claiming.

On Monday, Defense Minister Valeriy Heltey wrote on his Facebook page that Russia is making threats of a nuclear attack against the country if the war continues.

“The Russian side has threatened on several occasions across unofficial channels that, in the case of continued resistance they are ready to use a tactical nuclear weapon against us,” Heletey wrote.

The conflict between Russia and Ukraine has escalated rapidly after more than 1,000 Russian troops poured across the border last week. Fighting has picked up between both sides, and there are reports that Russian troops massacred hundreds of Ukrainian soldiers who were trying to retreat.

The alleged mass killing took place outside a besieged city about 22 miles east of Donetsk. Reports say that Russian troops initially agreed to let the Ukrainian troops retreat, but then reneged and opened fire, leaving hundreds of bodies littered along an agreed retreat route.

Lt. Col. Nikolai Gordienko, of the Ukrainian National Guard, said: “They were given the corridor to exit and they were shot. It is a violation of international conventions.”

The international community has condemned Russia’s actions and undertaken a series of sanctions meant to squeeze them out with economic pressure. Lithuanian President Dalia Grybauskaite has even claimed that Russia is effectively waging war against the entire EU.

“It is the fact that Russia is in a war state against Ukraine. That means it is in a state of war against a country which would like to be closely integrated with the EU. Practically Russia is in a state of war against Europe,” she said.

“That means we need to help Ukraine to… defend its territory and its people and to help militarily, especially with the military materials to help Ukraine to defend itself because today Ukraine is fighting a war on behalf of all Europe,” the leader of the former Soviet state said.

But amid the pressure and reports that Russia is making nuclear threats against Ukraine, Vladimir Putin has remained unmoved. This week he reportedly told outgoing European Commission President Jose Manual Barroso that he could “take Kiev in two weeks” if he wanted, Italian media reports have said.