The Russian government has agreed to allow the Polish authorities to examine Soviet archives on what Poles call the Katyn Forest massacre of 1940.
More than 20,000 Polish soldiers were killed by the Soviet secret police at the start of World War II.
Russian Foreign Minister Sergei Lavrov, hosting his Polish counterpart, said he hoped the two sides would be able to compare accounts of the incident.
Polish experts began an investigation into the killings earlier this month.
An official Russian report released in September avoided calling the deaths a war crime or a crime against humanity, which could still trigger prosecutions.
The massacre — in Katyn Forest near the Russian city of Smolensk — happened while the Poles were held prisoner.
Some families of the victims want the killings to be recognised as genocide.
The Polish Foreign Minister, Wlodzimierz Cimoszewicz, said he hoped the opportunity to examine the archives would not affect current ties between Moscow and Warsaw.
The Katyn massacre has been a sensitive issue in relations between Russia and Poland for more than a decade since the fall of communism in both countries.
The Nazis discovered the bodies of several thousand Polish officers at Katyn in 1943, but Moscow did not admit responsibility for the killings until 1989.
Source: BBC News