Nikolai Zelenschikov told the ITAR-TASS news agency that no US experts had been invited to take part in the inquiry, as "that will create difficulties in the work."

He did not elaborate, but recalled that NASA had not invited Russian experts to investigate the reasons of the Columbia shuttle crash.

"We shall definitely inform our US colleagues of the results of the work, we shall provide complete information, however so far there is nothing to provide", said Zelenschikov, who is first deputy designer general of the Energiya space corporation, which builds Soyuz spacecraft.

The inquiry's final report is to be delivered on May 23.

The US-Russian trio made a much steeper than expected uncontrolled entry for their landing Sunday on the plains of Kazakhstan after an apparent computer malfunction onboard forced them to abort the automatic landing.

Their Soyuz landed nearly 500 kilometres (310 miles) off target in the Kazakh steppe, and rescue services could not locate US astronauts Kenneth Bowersox and Donald Pettit and Russian cosmonaut Nikolai Budarin for more than two hours.

Because of the uncontrolled ballistic entry, the crew had a very rough landing, enduring the crushing weight of more than eight times the force of earth's gravity, that made it difficult for them to breathe.

The astronauts Tuesday denied precipitating their off-target landing by pressing the wrong button amid reports that their Soyuz TMA-1 craft -- a new, modified spaceship -- may accidentally have gone into reverse docking mode.

The inquiry chief implictly acknowledged that crew error was not to blame although he said no final conclusions could yet be made.

"There are several versions" of the cause of the botched landing, he told

"The crew is not under reproach for now, we have questions about the technical side and we are working on them," Zelenschikov said.

Russian spacecraft now provide the only transportation to the space station and Sunday's flight back from the ISS was the first return from space since the Columbia tragedy.

Columbia disintegrated during re-entry to the Earth's atmosphere on February 1, killing all seven crew members and leading NASA to suspend all shuttle missions, including those to the International Space Station.

SPACE.WIRE