Ukraine President Petro Poroshenko appealed to Russia's Vladimir Putin while Washington threatened tougher measures should Moscow fail to rein in separatists mounting.
The White House said it was "monitoring closely" events in Greece, where a new hard-left government has vowed to reopen the terms of a massive financial bailout.
French President called on Turkey to take new steps towards the "truth" behind the mass killings of Armenians a century ago, saying "it is time to break the taboos".
Entitled "The Right of Correspondence," the exhibition is organised by Russia’s top rights group, Memorial, and displays hundreds of letters sent by Gulag.
President Vladimir Putin slammed what he called attempts to rewrite history and lauded the Red Army at a ceremony marking the 70th anniversary of the liberation of Auschwitz.
The White House said it hoped to work closely with Greece's new government after the radical left-wing Syriza party stormed to power on an anti-austerity platform.
A rocket attack on the Ukrainian city of Mariupol that killed 30 people at the weekend deliberately targeted civilians and the perpetrators must be brought to justice.