Two Russians and a US astronaut aborted a return to Earth on Friday when their space capsule failed to separate from the International Space Station.

'This situation has never occurred before,' a spokewoman at Russian Mission Control near Moscow said, as space officials scrambled to determine the cause.

Nasa astronaut Tracy Caldwell Dyson and two Russian crew mates climbed into a Soyuz capsule hitched to the station for the descent, but latches holding the craft to a docking port failed to open, the spokeswoman said.

The only other window for a second undocking attempt last night passed without a solution to the problem.

Russia's space agency chief Anatoly Perminov told reporters at Mission Control that Dyson, Alexander Skvortsov and Mikhail Korniyenko's return to Earth after nearly six months in space had been rescheduled for tomorrow.

The docking mechanism did not function because the station's computer sent a false signal indicating the hatch between station and capsule was not fully sealed, Perminov said.

He said technicians had found no problem with the seals, and suggested they were still puzzling over exactly went wrong.

A second undocking attempt 'could have gone ahead today, but we need additional time to make sure we have reliable information about the problem,' Perminov told a terse news conference at Mission Control.

'There is no point in rushing,' he said. Perminov refused to take questions, saying he did not want to fuel rumours.

The website of the US space agency NASA said the crew had sent down video and photographs of a small star-shaped gear with two broken teeth. Experts were trying to determine whether it was related to the problem.

Russian Mission Control and the US space agency NASA's Mission Control in Houston agreed the three crew members would go back to the space station and await a fresh undocking attempt tomorrow, NASA said.

Three other crew members, Russian Fyodor Yurchikhin and NASA astronauts Doug Wheelock and Shannon Walker, would remain aboard the station as planned after tomrrow's departure.

'I see no technical problem on the station or anywhere that would threaten the crew,' Perminov said.

Another Russian space agency official, Alexei Krasnov, said the landing should now take place tomorrow at about 9:20am.

Skvortsov, Korniyenko and Dyson boarded the space station on April 4 after a flight up together in the Soyuz TMA-18 craft, which will also be used for their return to Earth.

At least two mishaps have already marred their stay. Failure of one of two cooling systems in August set off equipment shutdowns. In July, a faulty radio link forced an unmanned cargo ship to delay docking for two days.

US space shuttles have delivered some astronauts to the station, but single-use Russian Soyuz craft will ferry all crews after the Nasa retires its shuttle fleet next -Reuters