Poland accuses Russia of delaying handover of Smolensk air crash data

"The Polish side will be waiting for information and explanations about the reasons hampering the Russian side in forwarding the appropriate documents," Mr Tusk said, according to the Irish Times. "Now, when the probe is entering its final phase, our co-operation is worse than at the beginning," he added.

A formal request questioning the reasons for the delay will be addressed to the head of the Russian MAK Interstate Aviation Committee.

"Several weeks ago I submitted a list of documents that are of interest to the Polish commission but which it has not yet received," Mr Miller said Monday.

The exact date of Mr Miller's visit to Moscow is not yet known. Meanwhile, Mr Tusk said he would personally meet Russian government leaders or President Medvedev, if necessary.

Attorney general Andrzej Seremet said during a press conference last week that Poland was promised a further ten volumes of documents to be provided in August and expressed "slight concern" about the lack of Russian response in the hand-over deal.

Russia's initial cooperation after the crash near Katyn was seen as a possible new beginning in the relations between Warsaw and Moscow and was welcomed by Polish leaders.

But local Russian authorities since then have appeared to delay the passing of some documents and evidence from the aircraft crash site, causing annoyance in Warsaw.

In May, Mr Miller, accompanied by the attorney general and chief military prosecutor, went to Moscow to negotiate access to the black boxes storing recordings of pilots' conversation before the crash.

A spokesman for the Polish prosecution service said before the visit its main purpose was "to get assurances that the investigation materials will be transmitted to Poland as soon as possible."