Ukraine's decision to cave in to Russia by giving up its nuclear weapons in 1994 was a mistake, president Petro Poroshenko told Fox News in an interview that aired Monday.

Ukraine inherited its nuclear arsenal, the third-largest in the world, after it broke away from the Soviet Union in 1991. Three years later, Ukraine agreed to give up the material in a deal with the U.S., Russia and the United Kingdom. Under that agreement, each nation involved was supposed to respect the others' boundaries and sovereignty.

Speaking to "Special Report" anchor Bret Baier on Friday, Poroshenko admitted other nations could see Ukraine as an example of what not to follow. Russia and Ukraine are locked in a tug-of-war after Moscow took over Ukraine's Crimean Peninsula in 2014. Activists say Moscow has demonstrated a long-standing support for a separatist insurgency in eastern Ukraine.

"If you do not develop [a] new effective mechanism to stop [Russian aggression], we should forget about the non-proliferation process," Poroshenko said.

Analysts say Ukraine's arsenal in the early-1990s included nearly 2,000 Soviet strategic nuclear warheads.

Poroshenko says 6,000 Russian troops occupy eastern Ukraine, with an additional 50,000-plus troops in Crimea and 40,000 militia forces headed by Russian commanders.

"The global security system, which was built... post-World War II, is not working at all," he added.

Poroshenko also said nobody could predict Russian president Vladimir Putin's next global moves. "I don't trust him."

Over the weekend, Poroshenko responded to the series of "Panama Papers" leaks indicating that he set up an offshore holding company to move his candy business to the British Virgin Islands, possibly depriving Ukraine of millions of dollars tax revenue. Poroshenko insists he has done nothing wrong and hasn't managed his assets since being elected.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.