JOIN USRoscosmos said on its website that fragments of the stranded Phobos-Grunt voyager would probably fall to Earth today between 1436 GMT and 2224 GMT.
But it cancelled its earlier 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 9:51 pm (1751 GMT).
The unmanned USD 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.
It is loaded with enough toxic fuel to take it to the mysterious Mars moon Phobos and is weighed down further 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 taken 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 expects 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.