Russia's space agency on Sunday called off all predictions of the likely crash site of its ill-fated Mars probe only hours before the 13.5-tonne spacecraft was due to begin its fatal descent.

Roscosmos said on its website that fragments of the stranded Phobos-Grunt voyager would probably fall to Earth on Sunday between 1436 GMT and 2224 GMT.

But it cancelled its Saturday forecast of the debris splashing down in the Pacific off the western coast of Chile. Two earlier updates had the fragments falling into the Indian and Atlantic Oceans.

"The operations support group is keeping continuous watch of the Phobos-Grunt spacecraft's descent from orbit," the brief Roscosmos statement said.

The ITAR-TASS news agency cited space agency sources as saying the descent was now likely to begin at 10:40 pm (1840 GMT) and possibly see the debris scatter over a remote spot in the Chinese portion of the Gobi Desert.

The unmanned $165 million vessel -- stuck in a low Earth orbit since its November 9 launch -- will be one of the largest objects to re-enter the atmosphere since Russia brought down the Soviet-era Mir space station in 2001.

Sky gazers report the gold-coloured vessel emitting a bright orange glow as it traverses the globe in an eastward direction between London to the north and New Zealand to the south.

The Soviet-designed system is loaded with enough toxic fuel to take it to the mysterious Mars moon Phobos and is also weighed down by a Chinese satellite it had been due to put in orbit around the Red Planet under a landmark deal with Beijing.

Both Roscosmos and US space officials at NASA have made great efforts to downplay the dangers posed by the crashing probe.

Roscosmos predicts that only 20 or 30 segments weighing no more than 200 kilogrammes (440 pounds) combined will survive the fiery re-entry and actually hit the Earth's surface.

The chief NASA scientist on orbital space debris meanwhile dismissed the threat posed by the craft's toxic fuel.

Nicholas Johnson said the Russian craft's fuel tanks are made of aluminum rather than the much tougher titanium metal preferred by US space officials.

"According to Mr. Johnson, aluminum has a lower melting point than titanium and that significantly reduces the chances of the propellant reaching the surface of the Earth," NASA reported on its website.

ITAR-TASS said Roscosmos expected the fuel to burn up at an altitude of 100 kilometres (about 60 miles).

The inglorious ending provides a bitter reminder for Russia of the prowess it has lost in the half-century since Yuri Gagarin's historic first space shot in 1961.

The ambitious project had initially aimed to revive Russia's interplanetary programme and prepare the way for a manned mission to Mars.

But Russia lost sight of the probe almost immediately after its launch and then spent weeks trying to send commands that would either nudge it on its way to Mars or at least enable its controlled return to Earth.

Neither proved successful and Roscosmos now admits that the impact location will depend on varying circumstances such as atmospheric density and even solar activity.

The November accident represents one of the more high-profile mishaps in a year littered with unprecedented setbacks for the once-vaunted programme.

It struck less than three months after an unmanned Progress supply ship bound for the International Space Station crashed into Siberia.

Russia also lost three navigation satellites as well as an advanced military satellite and a telecommunications satellite in the past year.