The US military says it has no plans to shoot down debris from an out-of-control Chinese rocket that is barreling towards Earth.

The Long March 5B rocket was launched last week carrying the main part of China's first permanent space station.

The largest section of the used-up rocket — 30 metres long — is now expected to plunge back to Earth as early as Sunday afternoon AEST, at an unknown location.

US Defence Secretary Lloyd Austin said on Friday that there was no plan to shoot the rocket down.

"We're hopeful that it will land in a place where it won't harm anyone, hopefully in the ocean or someplace like that," he said.

Under international guidelines, rockets are supposed to return to Earth in a controlled way.

But space experts say that does not appear to be happening in this case, and China's space agency hasn't said if the rocket is under control or not. 

Basic details about the rocket stage and its trajectory are all unknown because of the lack of communication from the Chinese government. 

"For those of us who operate in the space domain there is a requirement — there should be a requirement — to operate in a safe and thoughtful mode and make sure that we take those kinds of things into consideration as we plan and conduct operations," General Austin said.

Chinese state-owned media outlet The Global Times described reports that the rocket was "out of control" and could cause damage as "Western hype".

Weighing around 22.5 tonnes, the rocket is one of the most massive pieces of space junk to make an uncontrolled re-entry into the atmosphere.

Moving at an average of 7 kilometres per second, its orbit covers an area between 42 degrees north and south; that's as far north as Boston in US, and as far south as Hobart.

The nonprofit Aerospace Corp said it expected the debris to hit the Pacific Ocean near the equator after passing over eastern US cities. 

Last May, another Chinese rocket fell uncontrolled off West Africa.

It landed in the Atlantic Ocean, but debris from the rocket landed near a village on the Côte d'Ivoire. Luckily, there were no casualties.

Since then, the space community has been speculating about whether the same thing could happen again, said space journalist Andrew Jones, who follows the Chinese program.

“We were wondering this time whether they would do the same thing again where they have an uncontrolled re-entry, or they had upgraded the booster [so it was capable of a controlled descent]," Mr Jones said.

"It seems [the upgrade] didn't happen. Whether that was simply that they didn't make any changes or they did make some changes and they didn't work, we don't know, because we haven't heard anything."

ABC/wires