Polish PM Donald Tusk has criticised an investigation by Russia into a deadly plane crash which killed the country's president in April.
He said it was "unacceptable" and some conclusions "without foundation".
Russia recently handed Poland a draft report of the incident after months of investigation, though it has not been made public.
Former President Lech Kaczynski and other senior officials were among 96 people killed in the crash.
The plane came down near the western Russian town of Smolensk.
Without revealing details of the report, Mr Tusk said it did not comply fully with the Chicago Convention which regulates international air travel.
"From the Polish point of view, the draft report from the Russian side as it has been sent is without question unacceptable," he said in televised comments to reporters in Brussels.
"This negligence and mistakes or lack of positive reaction to what Poland has been asking for, all these things allow us to say that some of the report's conclusions are without foundation," he added.
Russian President Dmitry Medvedev visited Poland last week and reiterated a promise to cooperate over the crash investigation.
Poland's first couple - along with other leading political and military figures - were on their way to a memorial ceremony for the World War II Katyn massacre when their plane crashed in poor weather on 10 April.
Russian Prime Minister Vladimir Putin was praised for his sensitive handling of the situation in the aftermath of the crash, which killed all those on board.