A decommissioned NASA satellite has crash-landed on earth after it fell uncontrollable through the atmosphere, but the precise location of the crash is not yet known, NASA said early on Saturday.

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

NASA said its decommissioned Upper Atmosphere Research Satellite (UARS) was "passing over Canada and Africa as well as vast portions of the Pacific, Atlantic and Indian oceans" as it returned to Earth.

The tumbling motion in which the satellite fell makes it hard to predict the precise location of the crash.

There were reports on Twitter of debris falling over Okotoks, a town south of Calgary in western Canadam, however. This remains to be confirmed.

The two dozen parts of the UARS that may have survived re-entry could weigh anything from two to 350 pounds (1-158 kilograms), the space agency said, and the debris field is expected to span 500 miles (800 kilometres).

It is likely to include titanium fuel tanks, beryllium housing and stainless steel batteries and wheel rims.

NASA has emphasised that the risk to the public is “very remote.” The space agency said that the chance of someone being hit by the debris was 1 in 3200, as most of the planet is covered in water and uninhabited land.

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

"Keep in mind we have bits of debris re-entering the atmosphere every single day."

They have also urged anyone who finds suspected debris to contact authorities rather than touch the debris.

Space law professor Frans von der Dunk from the University of Nebraska-Lincoln told AFP that the United States will likely have to pay damages to any country where the debris falls.

"The damage to be compensated is essentially without limit," von der Dunk said, referring to the 1972 Liability Convention to which the United States is one of 80 state signatories.

The UARS was launched in 1991 as an orbital observatory to study the Earth’s atmosphere, especially the ozone layer. It was decommissioned in 2005 due to lack of funding.

The satellite flew over most of the planet, travelling between 57 degrees north and 57 degrees south of the equator.

UARS was one of about 20,000 pieces of space debris in orbit around Earth. Something the size of UARS falls back into the atmosphere about once a year.