NASA officials scrambled Saturday to locate any remains of a bus-sized satellite -- the biggest piece of US space junk to plummet to earth in 30 years -- that disintegrated upon on re-entry into the Earth's atmosphere.

NASA has said there is only a "very remote" risk to the public from any of the fragments of the 6.3 tonne Upper Atmosphere Research Satellite (UARS) that may have survived the journey back into the atmosphere.

The satellite fell back to Earth between 11:23 pm Friday and 1:09 am Saturday (0323-0509 GMT Saturday), but the precise re-entry time and location "are not yet known with certainty," NASA said.

The tumbling motion of the satellite has made it difficult to narrow down where it landed, with the ocean considered likely and the exact number of pieces of debris it broke into is still unknown.

The Joint Space Operations Center at Vandenberg Air Force Base in California said the satellite penetrated the atmosphere over the Pacific Ocean, but the landing site was still not confirmed.

The 26 satellite fragments that NASA earlier said may survive re-entry could weigh anything from two to 350 pounds (1,158 kilograms). NASA said the debris field could span 500 miles (800 kilometers).

Canada, Africa and Australia had all been named as possible sites for touchdown of satellite debris.

On its Twitter feed, NASA said, "We're standing by for UARS location updates from the US Joint Space Operations Center. No reports of any damage or injury."

A media briefing was due at 2pm (1800 GMT).

Mark Matney, an orbital debris scientist at NASA, said earlier: "In the entire 50-plus year history of the space program, no person has ever been injured by a piece of re-entering space debris."

The Federal Aviation Administration issued a notice Thursday to pilots and flight crews, urging them to report any falling debris and take note of its position and time.

On Friday, Italy's civil protection agency warned that the probability of a crash in its northern territory had risen from 0.6 to 1.5 percent, and urged residents to stay indoors, on lower floors, preferably near load-bearing walls.

Orbital debris experts say space junk of this size from broken-down satellites and spent rockets tends to fall back to Earth about once a year, though this is the biggest NASA spacecraft to fall since the 85-ton Skylab crashed into western Australia in 1979.

The surviving chunks of the UARS -- launched in 1991 and decommissioned in 2005 -- could include titanium fuel tanks, beryllium housing and stainless steel batteries and wheel rims. The craft contains no fuel.

The US space agency has warned anyone who comes across what they believe may be UARS debris not to touch it but to contact authorities for assistance.