Polish officials are demanding Russia turn over documents they say are needed for their probe into the April plane crash that killed President Lech Kaczynski.
Prime Minister Donald Tusk said Monday that with the investigation entering its final phase, cooperation with Russia is worse now than it was at the start.
Poland's Gazeta Wyborcza newspaper quotes lead investigator Edmund Klich as saying Russia does not want to make the material available.
The newspaper says the documents in question could either confirm or rule out any part that Russian air traffic controllers and their equipment may have played a role in the crash.
The April 10 crash near the Russian city of Smolensk killed President Kaczynski and 95 others.
Foggy weather is believed to have played a part in the crash.
But investigators are looking into whether Mr. Kaczynski or someone else on board pressured the pilots to attempt a landing despite the bad conditions.
The president and his party were on their way to ceremonies marking the 70th anniversary of the Katyn massacre of Polish officers by Soviet secret police in World War II.
Some information for this report was provided by AFP.
