A row has broken out over claims by Polish investigators that credit cards were stolen from one of the passengers killed when President Lech Kaczynski's plane crashed in Russia in April.
About 6,000 Polish zloty (£2,000) was withdrawn on the cards, investigators say.
Polish officials say several Russian policemen or soldiers have been arrested.
But Russia has denied that both the theft and the arrests took place.
On Monday Monika Lewandowska, a spokeswoman for Warsaw prosecutors, said that the first withdrawal on the two stolen cards was made about two hours after the crash, in the city of Smolensk, where the Polish government plane crashed on 10 April.
She said other withdrawals took place over the next three days.
The cards belonged to Andrzej Przewoznik, a historian who oversaw wartime memorials in Poland.
He was one of a high-level delegation of Poles on their way to commemorate the 1940 Katyn massacre of more than 20,000 Poles by Soviet forces.
'No basis in reality'
A Polish government spokesman said on Sunday that three members of Russian special security police "who committed this disgusting act were quickly arrested thanks to co-operation between the Polish Security Agency and the Russian special services".
On Monday, a spokeswoman for Poland's Internal Security Agency said Russia had confirmed that four soldiers had been arrested.
But this was denied by Moscow. The Russian interior ministry rejected the reports as having "no basis in reality".
"No member of the Smolensk police has been arrested in connection with the actions described and published by the media," the ministry said.
Previously tense relations between Russia and Poland were widely considered to have undergone a dramatic change for the better following the trauma of the crash.