Prime Minister Vladimir Putin and his Polish counterpart, Donald Tusk, will attend a memorial service in April to commemorate the massacre of 20,000 Polish officers by Soviet forces during World War II.

The joint visit to the Katyn forest will mark a positive shift in relations for the countries, which have been at loggerheads over the actions of Soviet leader Josef Stalin in 1939, when he clinched a nonaggression pact with Nazi Germany that opened the way for the invasion of Poland and world war.

"Prime Minister [Putin] invited him to jointly visit Katyn," Putin's spokesman Dmitry Peskov said by telephone.

Tusk's office confirmed in a government statement that he would attend the memorial of the 1940 massacre of the Polish officers in the forest of Katyn and elsewhere.

Tusk's official spokesman told reporters in Warsaw: "Prime Minister Putin underlined that he is aware of how important the Katyn issue is for Poles. Prime Minister Putin said, 'I believe that our joint appearance at this ceremony will have a very big symbolic meaning.'"

Arguments between Russia and the West about who was responsible with Adolf Hitler for the start of World War II cast a shadow over last year's 70th anniversary commemorations in Poland and still irk relations.

Plans to install a U.S. anti-missile shield on Polish soil have also been a thorn in their side.

Even though former President Boris Yeltsin admitted in 1992 that his country was to blame for the Katyn massacre, Poland wants more to be done.

Poland demands the opening of archives related to an investigation — carried out between 1990 and 2004 — of the massacre, as well as an official rehabilitation of the victims.

In September, a senior Polish bishop, visiting the site of one of the graveyards in Katyn, said Poland must forgive Russia for Soviet crimes in order to improve relations.