Russia's space agency has adjusted its forecast for the crash of a failed spacecraft, saying it may shower its fragments into the south Atlantic.

Roscosmos said the unmanned Phobos-Grund probe could plummet to Earth Sunday or Monday anywhere along a broad swath between 51.4 degrees north and 51.4 degrees south.

It said Friday that the mid-point in the two-day window would have the craft crashing into the ocean about 400 kilometres east of the coast of Chubut province in southern Argentina.

It said the precise time and place of the uncontrolled plunge can only be clarified later as the probe draws closer to Earth.

At 13.2 tonnes, the Phobos-Grund is one of the heaviest pieces of space junk ever to plummet to Earth, with highly toxic fuel accounting for the bulk of its weight.

Meanwhile, the International Space Station has moved up and out of the way of a softball-sized piece of space junk.

NASA officials said debris from an old U.S. private communication satellite would have come within five kilometres of the orbiting outpost on Friday had the station not changed its orbit.

Space junk moves so fast that it can puncture the station, so engineers try to give debris a wide berth whenever something comes close.

Astronauts fired the station's engines for 54 seconds Friday to move the outpost about 305 metres higher. NASA said it needed to boost the station's orbit anyway to get ready for a cargo delivery.

This is the 13th time the space station has dodged space junk since 1998.