MOSCOW — Prime Minister Vladimir V. Putin joined his Polish counterpart on Wednesday in the first joint commemoration marking the anniversary of the murder of thousands of Polish officers by the Soviet Union at the beginning of World War II.

Mr. Putin met with Donald Tusk, the Polish prime minister, in Russia at a site in the Katyn forest close to the city of Smolensk, where 70 years ago members of the Soviet secret police executed over 20,000 Polish officers captured after the Soviet Red Army invaded Poland in 1939.

The circumstances surrounding the massacre have long been a major source of tension between Poland and Russia, and Wednesday’s tribute appears to be the latest step in an effort by both countries to patch up relations.

“The anniversary of the crimes at Katyn are very important for the Polish people,” said Robert Smigielski, an analyst with the Polish Institute of International Affairs in Warsaw. “You can say that it is the most problematic issue in relations between the Poles and the Russians.”

Only in the waning days of the Soviet Union did Moscow officially acknowledge the country’s role in the massacre, nearly half a century after the murders occurred. The Soviet government suppressed all information about the killings, blaming Nazi soldiers for the crime.

Still, some Russian leaders have continued to deny Soviet responsibility for the murders, even though, under President Boris N. Yeltsin in the 1990s, Russia released archival documents showing that Stalin’s Politburo directly ordered the massacre in March 1940. And Mr. Putin’s hesitation to publicly apologize for the murders and declare them war crimes has rankled many in Poland.

Russians in turn have been angered by Polish accusations of atrocities carried out by Red Army soldiers in Poland during World War II, and by attempts in Poland and elsewhere to equate Soviet actions during the war with Nazi atrocities. Germany and the Soviet Union had effectively divided Poland between them as part of a nonagression pact just before the war began.

Russian officials, including Mr. Putin and President Dmitri A. Medvedev, have lashed out at what they consider falsifications of history meant to sully the Soviet Union’s role in World War II.

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