By Adam Easton
BBC News, Warsaw
The Katyn massacre has long soured Russian-Polish relations
Russian PM Vladimir Putin has invited his Polish counterpart, Donald Tusk, to a ceremony to mark the 70th anniversary of the Katyn massacre.
It is the first Russian ceremony to mark the murdering by Soviet secret police of more than 20,000 Polish prisoners of war in April 1940.
The invitation is being hailed in Poland as a breakthrough that could lead to improved bilateral ties.
Mr Putin said he understood the significance of the massacre to Poles.
He told Mr Tusk in a telephone call that their joint appearance at the ceremony in April would be an important symbolic gesture, said a Polish government spokesman.
A former Polish foreign minister, Adam Rotfeld, who now heads a committee tackling difficult issues between the two countries, hailed Mr Putin's invitation as an important event in the normalisation of Polish-Russian relations.
The mass execution of Polish army and police officers in the forests of Katyn and other sites has long been one of the most difficult issues between the two countries.
For half a century the Soviet Union blamed the killings on the Nazis.
In 1990, Russian President Mikhail Gorbachev finally admitted Soviet responsibility.
More recently, Moscow's refusal to declassify the archives, and a Russian court ruling that the massacre did not warrant the term genocide, has angered many in Poland.