The unmanned Phobos-Ground probe fell Sunday after being stuck in Earth's orbit for two months.

The $170 million craft was one of the heaviest and most toxic pieces of space junk ever to crash to Earth, but space officials and experts said the risks posed by its crash were minimal because the toxic rocket fuel on board and most of the craft's structure would burn up in the atmosphere high above the ground anyway.

News agencies had cited Defense Ministry spokesman Alexei Zolotukhin as saying Sunday that fragments of the craft fell in the Pacific Ocean off Chile's coast. But Zolotukhin told The Associated Press Monday that estimate was based on calculations, and no witness reports had been received.

The deputy head of Russia's space agency, Anatoly Shilov, told state news channel Vesti that agency data assumed the craft broke up somewhere over Brazil.

A statement Monday from the space agency, Roscosmos, cited the reported Defense Ministry assessment, but gave no further information, noting "the lack of means of visual and other monitoring" in the region.

The Phobos-Ground probe was designed to travel to one of Mars' twin moons, Phobos, land on it, collect soil samples and fly them back to Earth in 2014 in one of the most daunting interplanetary missions ever. It got stranded in Earth's orbit after its Nov. 9 launch, and efforts by Russian and European Space Agency experts to bring it back to life failed.

The probe weighed 13.5 metric tons (14.9 tons), and that included a load of 11 metric tons (12 tons) of highly toxic rocket fuel intended for the long journey to the Martian moon of Phobos and left unused as the probe got stranded in orbit around Earth.

Roscosmos had said that all of the fuel would burn up on re-entry.

Phobos-Ground was Russia's most expensive and the most ambitious space mission since Soviet times. Its mission to the crater-dented, potato-shaped Martian moon was to give scientists precious materials that could shed more light on the genesis of the solar system.

Russia's space chief has acknowledged the Phobos-Ground mission was ill-prepared, but said that Roscosmos had to give it the go-ahead so as not to miss the limited Earth-to-Mars launch window.