Poland- Russian Prime Minister Vladimir Putin rebuffed criticism on Tuesday of Moscow's role just before World War Two during ceremonies marking the 70th anniversary of Nazi Germany's invasion of Poland.
But Putin and Polish Prime Minister Donald Tusk agreed that their countries' historians should work more closely to uncover darker parts of their shared past which still cloud relations 20 years after the collapse of communism in eastern Europe.
Russia and former satellites such as Poland are at loggerheads over the role of Soviet dictator Josef Stalin in 1939, when he clinched a non-aggression pact with Nazi Germany that opened the way for the invasion of Poland and world war.
If we are going to speak objectively about history we must understand it does not have just one color. It was diverse and a huge number of mistakes were made by all sides, Putin told a news conference after talks with Tusk in the resort of Sopot.
And all these actions created the conditions for the large scale aggression by Nazi Germany.
Russians are deeply proud of their country's victory over Hitler in 1945, but Poles, Balts and others say Stalin also bears direct responsibility for the outbreak of war, for carving up Poland with Hitler and also annexing the Baltic states.
Putin cited efforts by Britain and France to appease Hitler in 1938, resulting in their acceptance of the destruction of Czechoslovakia, as well as Poland's own seizure of a strip of Czech territory shortly before it too faced German invasion.
Putin and Tusk agreed to set up joint teams of historians to study the murder of Polish officers in a forest at Katyn in the western Soviet Union in 1940 -- an event which for Poles symbolizes what they see as Stalin's treachery and cruelty.
Poland wants Russia to apologize for Stalin's decision to have the entire Polish officer corps shot. For decades, Moscow blamed the deaths on the Nazis, but after the fall of the Soviet Union it acknowledged they had been shot on Stalin's orders.
At a ceremony held before dawn on Tuesday at Westerplatte on the Baltic coast, where the Germans fired the first shots against Poland on September 1, 1939, Polish President Lech Kaczynski compared Katyn to the Nazi genocide against the Jews.
There's one thing linking those crimes, though their scale was different. Jews perished because they were Jews. Polish officers perished because they were Polish officers, he said.
Tusk's center-right government is keen to improve ties with Russia, an important trade partner which provides nearly all of Poland's oil and almost half of its natural gas.
Tusk said he hoped Poland would agree a contract with Russia's Gazprom on new supplies of gas early this autumn. He also reiterated Poland's opposition to the Nord Stream project, which would transport Russian gas via the Baltic Sea to Germany.
On history, Tusk trod cautiously but firmly, stressing the need for honesty in facing up the past.
The truth can be painful but it should not humiliate anybody... If Prime Minister Putin's declaration means that for Russia, seeking the truth about those events is not a humiliation but a thought-out, wise strategy to reach an understanding for the future, then we are happy, he said.
Poles and other east Europeans are keenly awaiting Putin's speech on the war later in the day, though Moscow has made clear he will not apologize for Stalin's actions of 70 years ago.
German Chancellor Angela Merkel, the prime ministers of Italy and France and Britain's foreign secretary were among the guests attending Tuesday's commemorative events in Poland.
Poland lost about a fifth of its population, including the vast majority of its three million Jewish citizens, as well as a fifth of its territory during World War Two. After the war, it remained under Soviet domination until 1989.
Some 27 million Soviet citizens perished in the war after Hitler reneged on his pact with Stalin and invaded the Soviet Union in 1941.
(Writing by Gareth Jones; Editing by Charles Dick)