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China's Long March 5B rocket took off from the southern island of Hainan. (File photo: AFP) - 

A large segment of a Chinese rocket is expected to make an uncontrolled re-entry into the Earth's atmosphere on the weekend, but Beijing has downplayed fears and said there is a very low risk of any damage.

A Long March 5B rocket launched the first module of China's new space station into Earth's orbit on Apr 29. Its 18-tonne main segment is now in free fall and experts have said it is difficult to say precisely where and when it will re-enter the atmosphere.

Re-entry is expected to be around 11pm GMT on Saturday (7am on Sunday, Singapore time), according to the Pentagon, with a window of plus or minus nine hours either side.

Chinese authorities have said most of the rocket components would likely be destroyed on re-entry."The probability of causing harm ... on the ground is extremely low," Chinese foreign ministry spokesman Wang Wenbin told reporters on Friday.

Although there has been fevered speculation over exactly where the rocket - or parts of it - will land, there is a good chance any debris that does not burn up will just splash down into the ocean on a planet made up of 70 per cent water.

"We're hopeful that it will land in a place where it won't harm anyone," said Pentagon spokesman Mike Howard.

Defense Secretary Lloyd Austin earlier said that the US military had no plans to shoot it down, and suggested that China had been negligent in letting it fall out of orbit.

"Given the size of the object, there will necessarily be big pieces left over," said Florent Delefie, an astronomer at the Paris-PSL Observatory.

"The chances of debris landing on an inhabited zone are tiny, probably one in a million."

In 2020, debris from another Long March rocket fell on villages in the Ivory Coast, causing structural damage but no injuries or deaths//CNA