"The problem is that with all the great minds that focus on the issue, none can figure out what the problem is," NASA Administrator Michael Griffin told AFP in an interview.

"We don't know when we will know. Smart people are working on it," he said, noting that the National Aeronautics and Space Administration and Russia are closely cooperating on the issue. "We are doing the best we can."

"On both sides we have been unable to trace the cause," he said.

After leaving the International Space Station on April 19, the Russian capsule went off track, landing 420 kilometers (261 miles) from its target in the steppes of Kazakhstan.

The re-entry subjected US astronaut Peggy Whitson, South Korean counterpart Yi So-Yeon and Russian Yuri Malentchenko to experience uncomfortably high G forces.

A similar glitch took place during a previous landing in October.

Last month, two Russian cosmonauts went on a spacewalk outside the International Space Station to remove an explosive bolt that had been suspected of causing the irregular descents.

But Griffin said it was unclear whether the bolt was to blame.

NASA has a deep interest in the reliability of the Soyuz rocket since it will become its sole transport vehicle to the International Space Station after 2010, the year the US space agency retires its three space shuttles.

NASA is building its own capsule, but it will only be ready in 2014 or 2015.

The Soyuz is known for its reliability and has carried out more than 1,600 flights.

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