China has banned for 40 days civil aviation from a large airspace off Shanghai without specifying the reasons, according to a notice to aviators published by the US Federal Aviation Administration (FAA).
The aviation notice (NOTAM) announcing the access restriction was posted online on March 27 at 1150 GMT and took effect a few hours later.
It will remain in place until May 6 and affects an area twice the size of Taiwan a few hundred kilometres north of the island, covering 73,000sq km in total.
China has not provided any explanation for the ban as of April 8, but "there is no possible use other than military" for this type of airspace restriction, maritime security expert Benjamin Blandin told AFP.
"It could be to fire missiles, carry out air exercises, etc. We don't know," he added.
This is "the very first time" that China has restricted access to its airspace in a way that is "so sudden, geographically extensive, prolonged over time, and so poorly documented", according to Blandin, a researcher at Taiwan's Institute for National Defense and Security Research (INDSR).
Aviation and defence consultant for Aviation NXT, Xavier Tytelman, also believes that the restriction is "out of the ordinary" in terms of its size, duration and the lack of any limits on altitude, either high or low.
NOTAMs are intended to inform aviators of unusual circumstances affecting certain airspaces.
They are generally issued ahead of military exercises or during exceptional events such as fires or volcanic eruptions.
In this particular case, it means that "the government is reserving a zone for itself," said Tytelman.
Restricted access applies to civil aviation but not necessarily to military planes, helicopters or drones.
The ban covers two zones over the Yellow Sea, between China and South Korea, and three others straddling the Yellow Sea and East China Sea, between China and its neighbour Japan.
"Normally this is announced weeks or months in advance, and there are explanations and justifications," said Blandin, adding that the reserved airspace is limited in altitude "to allow the passage of commercial aircraft".
The areas closed to civil aviation are separated by an air corridor about 100 kilometres wide, allowing access to Shanghai from the Yellow Sea.
"Access denials"
According to a Taiwan senior security official, China is taking advantage of the United States being distracted by the Middle East war to expand its active military presence and conduct harassment in the Indo-Pacific.
China aims to deter the US's allies in the region and weaken the US's military influence in the Indo-Pacific, he said.
For Blandin this NOTAM is part of a "continuing series of access denials" and "bolsters China's strategy of nibbling away at the land and sea borders of its neighbours" over the past 15 years.
Tytelman, a former flight navigator for the French navy, expects this type of operation to be repeated in the months and years to come, in order to "lower our guard before an aggressive action."
