By Mick Krever, CNN

Ukraine is at the beginning of a “very dangerous conflict,” Ukrainian member of parliament and former foreign minister Petro Poroshenko told CNN’s Christiane Amanpour on Tuesday.

“Several weeks ago we had a guarantee that nothing [would] happen with the Crimea. Several weeks ago we had [a situation] that there is not any military presence on Ukrainian territory, including the Crimea.”

A Ukrainian officer was killed at a Crimean military base on Tuesday, and a second person injured, by armed men in masks.

Ukraine’s armed forces then announced that it had authorized units stationed in Crimea to use weapons “to protect and preserve the life of Ukrainian soldiers.”

Ukraine on edge of 'dangerous conflict' Click to watch video

“We have a feeling that we are at the beginning of a very dangerous conflict. And we should do our best to stop this process.”

The Kremlin now says that Ukraine’s Crimea region is part of Russia, and President Vladimir Putin signed a draft annexation agreement on Tuesday, which still needs the Duma’s rubber stamp.

Poroshenko, a self-made billionaire who runs a confectionary empire, tops opinion polls as a potential Ukrainian president. The country is set to hold elections to replace the interim government at the end of May.

“I strongly believe that this is not only Ukrainian territory is now threatened,” he said. “Now under attack can be any country in the European Union, including other parts of Ukraine. That’s why we should think that it can never happen again.”

After the death today of the Ukrainian officer in Crimea, Poroshenko said, the “cold period” of the conflict is over.

“The whole system of the European and global security is finished.”

After Crimea, 'global security finished' Click to watch video

Poroshenko called for a renegotiation of the 1994 Budapest Memorandum, in which the West and Russia gave some security assurances to Ukraine in exchange for that country’s relinquishing of the left-over Soviet nuclear weapons left on its territory.

“Are you asking for your nuclear arsenal back?” Amanpour asked.

“Not at all. We are responsible politicians. Ukraine is a peaceful country. But we are now under aggression.”

Poroshenko said that there had been absolutely no negotiation between Russian and Ukrainian officials.

No talks between Russia and Ukraine Click to watch video

“Until today we don’t have any…signs from the Russian side for the direct negotiations. And this is the behavior of the aggressor.”

“I am a former diplomat, I am a former minister of foreign affairs, and I think this is absolutely irresponsible, this way of providing the relationship between Ukraine and Russia.”

He asked for Western powers to help arrange such talks, and demanded that Crimea be de-occupied by Russian forces. The territory, he said, would always remain Ukrainian.

“We now should be responsible to renew this trust, and [to] renew this trust this is the one demand: De-occupy the Crimea and cancel what’s happened today.”