Any debris that survives will most likely hit water or ground within 20 minutes of the space station's entry into the Earth's atmosphere, Sean O'Keefe, a former NASA administrator and university professor at Syracuse University, told USA Today.

Anyone who sees what could be debris from the Heavenly Palace should not touch it or inhale its fumes, Aerospace Corp warned. The station carried a "highly" toxic substance called hydrazine.

The China Manned Space Engineering Office, attempting to assuage concerns that the crash could result in damage or deaths, said on Sunday that the space station would mostly likely burn up before it hits the Earth, the South China Morning Post reported.

The manned space office said the main structure would burn up from the heat and friction. Any debris that reaches the ground would be "floating down at a very slow speed due to their small mass", the report said.

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The space station is not likely to affect aviation activities, the report added.

Speaking at a daily news briefing on Friday, Foreign Ministry spokesman Lu Kang said the government had continually informed the UN space agency of the latest information about the Tiangong-1.

China had been responsible and transparent, Lu said.

"If there is a need, we will promptly be in touch with the relevant country," he said.

"As to what I have heard, at present the chances of large fragments falling to the ground are not very great, the probability is extremely small."

The 10.4-metre-long Tiangong-1, was launched in 2011 to carry out docking and orbit experiments as part of China's ambitious space program, which aims to place a permanent station in orbit by 2023.

The spacelab was originally planned to be decommissioned in 2013 but its mission was repeatedly extended.

China had said its re-entry into Earth's atmosphere would happen in late 2017 but that process was delayed, leading some experts to suggest the space laboratory is out of control.

Advancing China's space program is a priority for President Xi Jinping, who has called for Beijing to become a global space power with both advanced civilian space flight and capabilities that strengthen national security.

Reuters, USA Today