Ukrainian Defense Minister Valeriy Heletey announced Monday that Kiev has been threatened with a nuclear attack should it continue its battle against pro-Russian forces in eastern Ukraine.

According to Heletey’s statement on Facebook, “The Russian side has threatened on several occasions across official channels that, in the case of continued resistance they are ready to use a tactical nuclear weapon against us.” He continued by saying that a “full-scale invasion” by Moscow is in the works. He called for the Ukrainian military to “begin building defenses against Russia.”

The Ukrainian Defense Minister also accused Moscow of starting a “great war” that could end in the loss of tens of thousands of lives on both sides. “A great war has arrived at our doorstep, the likes of which Europe has not seen since World War II,” he said.

Separately, a top Russian official has said that Russia is changing its military strategy due to NATO’s increased activity in eastern Europe in hopes to protect Ukraine, the BBC reported.

Mikhail Popov, a top adviser at the Kremlin, said that “NATO’s planned action … is evidence of the desire of US and NATO leaders to continue their policy of aggravating tensions with Russia.” He said that NATO is considered to be a main “external threat” to Russia’s security interests.

Meanwhile, Italian Foreign Minister Federica Mogherini has called for NATO to take a more active role in supporting eastern European nations against Russia’s incursion. Mogherini said that NATO’s Article Five, which commits the 28 member states to defend each other should one come under attack, needs to be more than just “written text.”

NATO Secretary-General Anders Fogh Rasmussen said Monday, “We must face the reality that Russia … considers NATO an adversary. We cannot afford to be naive.”

NATO is hosting a two-day summit in Wales that starts Thursday. The member nations will discuss the threat posed by Russia to eastern Europe. They will also discuss strategies to contain the Islamic State terror group that is expanding its reach through Syria and Iraq.