Former Soviet leader Mikhail Gorbachev says tensions between the major powers have put the world "on the brink of a new Cold War".
He accuses the West, particularly the United States, of giving in to "triumphalism" after the collapse of the communist bloc a quarter of a century ago.
Gorbachev spoke today at an event marking the 25th anniversary of the fall of the Berlin Wall, close to the city's Brandenburg Gate.
Gorbachev called for new trust to be built through dialogue with Moscow, and suggested the West should lift sanctions imposed against senior Russian officials over its actions in eastern Ukraine.
He says failure to achieve security in Europe would make the continent irrelevant in world affairs.
Gorbachev's comments echoed those of Roland Dumas, France's foreign minister at the time the Berlin Wall fell.
"Without freedom between nations, without respect of one nation to another, and without strong and brave disarmament policy, everything could start over again tomorrow," Dumas said. "Even everything we used to know, and what we called the Cold War."
US president Barack Obama appeared to share some of Gorbachev's concerns for Europe, though he blamed Moscow for the current tensions.
Paying tribute to the East Berliners who pushed past border guards to flood through the Wall on November 9 1989, Obama said in a statement on Friday that "as Russia's actions against Ukraine remind us, we have more work to do to fully realise our shared vision of a Europe that is whole, free and at peace".