The South Korean and U.S. Air Forces believe that debris from a Chinese rocket will likely hit Earth some time on Sunday morning Korea time.
According to a South Korean military source on Saturday, the two sides are closely monitoring the situation, estimating the free fall the next day.
The South Pacific ocean near Australia was initially forecast as a possible crash site, but the Gulf of Mexico is also being mentioned.
U.S. Space Command is trying to track the possible site of impact, but said the exact location can't be pinpointed until within hours of the debris' re-entry into the atmosphere.
A Korean military source said predictions continuously change, but the Korean Peninsula is not within the trajectory, and added that Seoul and Washington were keeping close tabs and sharing surveillance data.
Last week, China launched the Long March 5B Rocket carrying a module of its first permanent space station into orbit. A rocket segment is due to plunge back to Earth in an uncontrolled re-entry.
Weighing over 22 tons, the object is not projected to hit any populated areas.