WARSAW (AFP) - The military pilots blamed for the deadly 2010 crash of a Polish presidential jet in Russia were pressured into landing in thick fog by their superior, according to new transcripts of cockpit recordings leaked on Tuesday.

The plane went down on April 10, 2010 as it was approaching Smolensk airport in western Russia, killing 96 people, including then president Lech Kaczynski, his wife, the central bank head and military chief of staff.

Transcripts from one of the plane's black boxes published by Poland's RMF FM radio station show that the then Polish air force commander General Andrzej Blasik was in the cockpit alongside the pilots.

"The fact is, we have to get it (the landing) done," Blasik told the pilots six minutes before the crash, repeating the words of a senior foreign ministry official who had been in the cockpit earlier.

"Go on, you've got enough room," Blasik told pilots, apparently urging them to tackle the rundown air strip when the plane was at a height of 300m.

Despite several attempts, pilots failed to evict "third persons" from the cockpit, according to the transcripts.

Warsaw military prosecutors investigating the crash issued a statement on Tuesday saying the leaked transcripts were "imprecise", but did not deny their authenticity.

They also confirmed that experts had recently submitted fresh transcripts which "decrypted 40 per cent more words" than previous ones.

Military prosecutors also said they had launched a probe into who leaked the transcripts, an offence carrying a penalty of up to two years behind bars.

The prosecutors said last month that pilot error was to blame for the disaster, but also charged two Russian air traffic controllers with triggering the crash.

Conspiracy theories still abound as to the causes of the crash as Poles prepare to mark the fifth anniversary of what is regarded as their country's worst peacetime disaster.

The late Kaczynski's twin brother, Jaroslaw, and members of the Law and Justice (PiS) opposition party believe the crash was part of a plot against the president, among the EU's most vocal critics of then Russian prime minister Vladimir Putin.

Russia has so far refused to hand over the plane's wreckage to Polish authorities, insisting its investigation is ongoing.

Warsaw has extended its investigation until October 10.

The Russian-made Tupolev was carrying a delegation headed for memorial ceremonies in Katyn for thousands of Polish army officers slain by the Soviet secret police in 1940, a massacre the Kremlin denied until 1990.