The Russian ambassador to Poland has sparked outrage for putting some of the blame for World War II on Poland, a spat that comes amid worsening ties between the Slavic nations.

The Polish Foreign Ministry said Saturday that the ambassador "undermined historical truth" and seemed to be trying to justify Stalinist crimes. Prime Minister Ewa Kopacz also strongly protested.

World War II began after Nazi Germany and the Soviet Union sealed a pact in 1939 that included a secret provision to carve up Poland. Germany first invaded Poland from the West, followed by a Soviet invasion from the east 16 days later. Millions of Poles were killed in the war.

Russian Ambassador Sergey Andreev said the Soviet Union's invasion of Poland in 1939 was not an aggression.