Space technicians are trying to work out why the Soyuz TMA-1 that brought Kenneth Bowersox, Donald Pettit and Nikolai Budarin back to earth Sunday landed 440 kilometres (275 miles) off course in the Kazakh desert amid fears that a malfunction could throw the launch programme into doubt.

The three men were originally to have left the International Space Station in March but their return was delayed after US spacecraft Columbia disintegrated on its return from the ISS on February 1, killing all seven crew on board.

Russian spacecraft now provide the only transportation to the space station.

The head of the Energiya Space Corporation Yury Semenov said that all possible causes of the inaccurate landing were being examined.

"We must examine all causes. ... There was a version (of events) that (flight commander) Nikolai Budarin pushed a button on the control panel," he told the ITAR-TASS news agency.

"I spoke with him (Budarin), but he had not touched anything," he noted.

The capsule's crew had not detected anything abnormal by the time the descent capsule separated from the Soyuz, Semenov added.

"There were normal conditions up to the time of the descent," he said.

The Energiya head said the causes of the capsule's failure to land in the targeted area should be known by next month.

Russian officials said earlier that the Soyuz, based on technology dating back more than three decades, had been forced to land using an uncontrolled ballistic re-entry rather than the usual manual or automatic procedure.

SPACE.WIRE