Due to Russian aggression Ukraine has tightened security measures at its nuclear power plants, Ukrainian President Petro Poroshenko has said.

"The Kremlin's refusal to recognize Ukraine's independence and the lack of effective security guarantees for a nuclear-weapon-free Ukraine in the Budapest Memorandum, in turn, triggered Russia's attack on our independent state. The Russian aggression firstly undermined the confidence of non-nuclear states in the regime of non-proliferation of these weapons and secondly created a threat of recurrence of the nuclear disaster on the territory of our state," Poroshenko said during a visit to Chornobyl on Tuesday.

He noted that the intense fighting took place just a few hundred kilometers from the Zaporizhia Nuclear Power Plant. "Russia was preparing a full-scale war against Ukraine and planned military operations and unrest in most parts of our country, including in the places where a lot of dangerous facilities are located," the president said.

Poroshenko said Ukraine tightened security measures at its nuclear power plants and protects them from possible attacks by terrorist sabotage groups.

"The fight against possible nuclear terrorism should be coordinated at the international level, and the State Security Service of Ukraine has been actively involved in this cooperation," the Ukrainian president said recalling that he had discussed the matters of physical protection of radioactive materials and nuclear facilities from possible sabotage at the recent Nuclear Security Summit in Washington.

"I stressed this at the summit and today I repeat that at the moment the Kremlin is deploying elements of its nuclear potential in the Russian-occupied Crimea. We and our partners have to decide how to improve the security guarantees from the nuclear powers to those countries that do not possess weapons of mass destruction," Poroshenko said.