The irregular landing of a Russian space capsule last week is not a "major problem," a top NASA official said, despite reports that the astronauts on board could have died.

"I don't see this as a major problem, but it is clearly something that should not have occurred," said Bill Gerstenmaier, associate administrator for Space Operations.

He added: "I think there is inherent reliability in this (Soyuz) system."

In an interview with a group of journalists Tuesday, whose transcript was obtained by AFP, he said NASA had heard nothing from the Russians suggesting the lives of the Russian, American and South Korean astronauts onboard were at risk.

"They're concerned about the event, but the relative danger to the crew, we've had no discussion on that at all," he said. "They've not conveyed to us or conveyed to me any concerns at this point."

Russian media said Tuesday the astronauts were lucky to survive Saturday's dangerous re-entry into the Earth's atmosphere, when their Russian-designed Soyuz landing capsule landed hundreds of kilometres (miles) off target.

"The fact that the crew members remained unharmed, in one piece, was very lucky. Everything could have ended much worse," Interfax news agency quoted an unnamed source close to the investigation as saying.

Gerstenmaier said the Russians had launched an investigation into the incident and he would wait for that, adding: "We need to get the capsule back to understand what occurred. I don't want us to speculate."

He said officials from NASA (National Aeronautics and Space Administration) would probably discuss the results of the probe with their Russian space agency counterparts before the next launch of the Soyuz TMA-12 in May.

"It may be a month or so before we start hearing anything definitive back from the commission," he added.

Meanwhile, in a transcript of an audio recording made after Saturday's landing, American astronaut Peggy Whitson is heard describing the descent as "pretty dramatic," adding that she was stuck in her seat the whole time.

The landing subjected the crew to huge gravitational forces, and Whitson said: "Gravity is not really my friend right now and 8Gs (a force eight times that of on Earth) was especially not my friend."

South Korean scientist Yi So-Yeon and Russian cosmonaut Yury Malenchenko were also on board, coming back from the International Space Station.

The Soyuz, long considered a reliable workhorse for human space travel, has had two similar incidents in recent years.