MOSCOW — Three astronauts began their return to Earth from the International Space Station on Friday aboard a Soviet-era capsule whose mission follows an unprecedented spate of Russian space accidents.

The Moscow-based Roskosmos space agency said the Soyuz TMA-21 craft undocked from the orbiter with two Russians and a US astronaut on board according to schedule at 0038 GMT.

"We have physical separation," the departing crew's commander Andrei Borisenko said in comments broadcast on NASA TV.

"You look great backing away guys," the next mission's commander Mike Fossum told the crew as it started its journey home over northern China. "It was great sharing space with you."

The small aerodynamic craft is due to touch down with the help of a massive parachute in the windy steppes of the ex-Soviet republic of Kazakhstan at 0400 GMT.

The Soyuz leaves behind a skeleton crew of three -- NASA's Fossum and Russia's Sergei Volkov along with the Japanese flight engineer Satoshi Furukawa -- on board the orbiter at one of the tensest times in its storied history.

Russia kicked off the international project's construction in 1998 in the midst of a heated race with the United States for space supremacy in the early post-Soviet era.

But the programme has recently suffered an embarrassing series of setbacks that saw sensitive satellites fail to reach orbit and -- in the most dramatic accident last month -- an ISS cargo craft crash back to Earth.

The August 24 accident prompted Russia to temporarily ground a part of its Soyuz programme while it conducted emergency checks.

Various versions of the Soyuz rocket are used to ferry both cargo ships and fresh crews to space. Its re-entry vessels remained docked to the station during missions and then used by crews to return to Earth.

This low-cost simplicity has been Russia's pride and made the Soyuz especially indispensable since the July retirement of the US space shuttle programme.

NASA had been mulling the option of leaving the space station abandoned for the first time in 10 years should Roskosmos fail to solve its problems by mid-November.

The Russian agency this week scheduled the next manned Soyuz mission for November 12 -- just days before NASA's final deadline.