China's Tiangong I space lab is expected to re-enter the atmosphere between Saturday and Monday, according to the China Manned Space Agency.

The latest announcement, posted on Thursday afternoon on the agency's website, said Tiangong I was still in orbit at an average altitude of 196.4 kilometers on Thursday, which means it had descended 5.9 km since Wednesday. It also narrowed the earlier predicted window from the Beijing Aerospace Control Center, which had estimated that the re-entry would take place between Saturday and Wednesday.

Sources close to the matter told China Daily that it is extremely difficult to predict the exact time and location of the spacecraft's re-entry due to a host of factors such as the unpredictability of upper atmospheric drag and the ultrafast speed of the spacecraft.

In the latest note sent on Monday by China's permanent mission to the United Nations in Vienna to the UN Committee on the Peaceful Uses of Outer Space, Chinese space authorities said Tiangong I's re-entry area will be located between 43 degrees north latitude and 43 degrees south latitude.

The note said most structural parts of the spacecraft will burn up during re-entry and "the probability of damage to aviation activities and human life and facilities on Earth is extremely low".

It added that China will continue to closely monitor the status of Tiangong I and provide information on its orbit and estimated time of re-entry through the China Manned Space Agency's website.

The 8.5-metric-ton Tiangong I, the country's first space lab, was launched by a Long March 2F carrier rocket at Jiuquan Satellite Launch Center in northwestern China in September 2011. The spacecraft had a designated life span of two years but was in service for four and a half years before its retirement was announced by the Chinese space authorities in March 2016.

During its operation, the space lab conducted six automatic and astronaut-controlled dockings with the nation's Shenzhou VIII, Shenzhou IX and Shenzhou X spacecraft.

China launched its second space lab, Tiangong II, in September 2016. It was boarded by two Chinese astronauts during the Shenzhou XI mission in October and November of that year.