The Kremlin's invitation to Poland's premier to attend events to mark the massacre of thousands of Polish POWs by Soviet forces in WWII is an important step in the improvement of bilateral ties, the senate speaker said on Thursday.

Russian Prime Minister Vladimir Putin invited his Polish counterpart Donald Tusk on Wednesday to attend a memorial service in April to mark the 1940 massacre. Tusk confirmed his participation in the event later in the day.

Bogdan Borusewicz said it was "an important signal from the Kremlin," and complicated bilateral ties "are increasingly improving." Borusewicz will visit Moscow on February 9 at the invitation of his Russian counterpart, Sergei Mironov.

Over 20,000 Polish officers, police and civilians taken prisoner during the 1939 partitioning of Poland by the Soviet Union and Nazi Germany were killed in western Russia's Katyn forest, as well as in prisons and other locations, by the NKVD, the forerunner of the KGB.

The Soviet Union acknowledged the massacre in 1990. Modern Russia also recognized Soviet responsibility for the mass shooting, but has not classified it as a war crime, something Warsaw has demanded.

The two Slavic nations have recently been at odds over Soviet leader Joseph Stalin's 1939 deal with Nazi Germany, which Warsaw says triggered the invasion of Poland and the start of WWII.

Putin moved to heal the rift over the massacre and the 1939 deal when he visited Poland in September to mark the 70th anniversary of its invasion. He called the killings "a crime" but urged "forgiveness." He said all European countries bear their share of responsibility for the outbreak of the war.

WARSAW, February 4 (RIA Novosti)