Gdansk (Poland) (AFP) - Ukrainian President Petro Poroshenko on Thursday criticised what he termed a policy of appeasement towards Russia in the context of the separatist conflict in Ukraine's east.

"Crimes are committed today in the 21st century amid the aggression against my country Ukraine, despite the cruelest lessons of the past," Poroshenko said in the Polish port city of Gdansk.

He was speaking at ceremonies marking the 70th anniversary of the end of World War II that were attended by leaders including UN Secretary-General Ban Ki-moon and EU President Donald Tusk.

"Astonishingly, we see attempts to appease the attackers," he continued, without specifying the events or people responsible for the policy.

Poroshenko also slammed Russia's upcoming May 9 World War II victory parade in Moscow as a "parade of cynicism".

"The European Union is facing the most difficult challenge in its history," Poroshenko said. "A test of its unity, its solidarity, its fundamental principles."

Europe and the United States have slapped the toughest sanctions since the Cold War on Moscow over allegations that it is behind the separatist rebellion in eastern Ukraine.

The country's warring parties tried to shore up a faltering truce deal Wednesday after Kiev said five soldiers were killed in clashes in the war-torn east.

The move came as US Defense Secretary Ashton Carter told lawmakers in Washington on Wednesday that pro-Russian separatists appeared to be making preparations for a fresh offensive.