Russia on Friday declassified documents that shed light on Yuri Gagarin's mysterious death in a training flight in 1968, saying his jet likely manoeuvred sharply to avoid a weather balloon.

Top Kremlin archives official Alexander Stepanov told a news conference that a Soviet-era commission -- whose conclusions had until now been classified -- has concluded that this was the most likely cause of his death.

"The conclusions of the commission are that the most likely cause of the catastrophe was a sharp manoeuvre to avoid a balloon probe," he said. Balloon probes are often used for weather-forecasting purposes.

But he indicated that the conclusions of the commission, whose documents were de-classified to mark the half century of Gagarin's voyage into space in April 1961, had given a second possible cause for the manoeuvre.

"A less probable cause was avoiding entry into the upper limit of the first layer of cloud cover," he added.

Stepanov said he hoped the publication would quash rumours about the cause of Gagarin's death.

"I hope they will dismiss very many speculations that are circulating in Russia in pseudo-history books," he said.

The conclusions of an investigation were outlined in a decree of the Central Committee of the Communist Party and the Council of Ministers dated November 28, 1968, which was marked "absolutely secret."

The presidential archive published the two-page conclusion of the commission, signed off by party chief Leonid Brezhnev, in a book of archive materials published for the 50th anniversary of Gagarin's first manned space flight.

The book, which has more than 700 pages, was published in a print-run of only 800 copies, the Russian space agency said in a statement after the news conference.

The book also includes the notes of the chief investigators, senior Soviet defence officials Leonid Smirnov and Dmitry Ustinov, the space agency said.

Gagarin died on March 27, 1968, when a MiG fighter jet carrying him and instructor Vladimir Seryogin crashed in the Vladimir region outside Moscow while on a training flight.

The commission writes that manoeuvres by Gagarin or Seryogin led the jet into a "supercritical flight regime and to its stalling in complex meteorological conditions."

The commission created straight after Gagarin's death included defence ministry officials and aircraft designers, Stepanov said. The commission studied the flight training of the pilots and the condition of the plane.

Over the years the death of the first man in space has been explained variously by the result of a rapid change in height to avoid an object, interference from another craft, or oxygen deprivation in the cabin.

There has been speculation of sabotage by conspiracy theorists although this has never been backed up by evidence.

Asked to comment on rumours that parts of the MiG are still stored in secrecy, Roskosmos deputy head Vitaly Davydov said he could not comment.

"Roskosmos has nothing to do with the investigation of air crashes, so this question has nothing to do with us. We are not indifferent to this, but I can't say anything."

Davydov stressed that Russia has nothing to hide about Gagarin's space flight and was not hiding any scandalous documents in secret archives.

"Today there aren't documents on Gagarin's flight left that we have consciously kept secret because it would be damaging if they were made public," he said.

He admitted that the Soviet authorities lied about the fact that Gagarin landed by parachute rather than inside a space capsule, as was officially announced.

"The question was whether the cosmonaut was in the capsule or not. Then for a number of reasons they took the decision to say he was and that was how the record was registered," he said.

He stressed that Gagarin would have set the space flight record in any case.

The Soviet authorities acted in this way "even though everyone understood what happened and there would not have been any problem to change the way the record was registered," he said.