China has mysteriously closed off five areas of airspace along its northeastern coastline for 40 days in a possible preparation for military action.
The restriction was put in place on March 27 and will remain in place until May 6.
Chinese authorities typically impose such limitations ahead of planned military exercises - yet no drills have been officially announced for this timeframe.
Military operations conducted by the People's Liberation Army (PLA) rarely extend beyond a short number of days, making this prolonged reservation highly unusual.
The closure has been issued as a "Notice to Air Missions", commonly referred to as Notams, which serves to alert aviation bodies to temporary dangers on flight paths or at particular locations.
According to the US Federal Aviation Authority, the restricted zones span roughly 340 miles, stretching from the Yellow Sea southward to the East China Sea near Shanghai.
The restriction carries no altitude ceiling, applying to all aerial activity - including spacecraft.
The closure coincides with the full duration of the Nasa-led Artemis II crewed lunar flyby mission, which lasts 10 days.
Chinese authorities typically impose such limitations ahead of planned military exercises
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The area lies several hundred miles to the north of Taiwan and encompasses a corridor frequently used by the PLA for military training.
Analysts speaking to the Wall Street Journal suggested the reserved airspace might enable Beijing to rehearse aerial combat tactics that could prove valuable should it attempt to invade Taiwan.
Such exercises could involve practising the creation of choke points along routes American forces might use to reach the island nation.
Washington maintains defence installations, combat units and tens of thousands of military personnel across South Korea and Japan, which experts described as the "tip of the spear" in any potential defence of Taiwan.
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Experts say it could allow Beijing to rehearse aerial tactics for a possible future invasion of Taiwan
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However, a senior Taiwanese official has said the airspace closure was likely "aimed at Japan".
The two nations have been locked in a diplomatic dispute since October.
A Japanese Self-Defence Force officer forced entry into China's Tokyo embassy last month, sparking fury from Beijing.
China claims sovereignty over the Senkaku Islands, which Japan controls, and regularly dispatches coastguard vessels to waters surrounding the territory.
Japan and China have been locked in a diplomatic dispute since October
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Defence ministry-affiliated experts in Japan have previously said a potential Chinese incursion into the Senkaku Islands ranks among the nation's most urgent security threats.
The airspace restriction coincides with a Chinese visit by Chen Li-wun, who leads Taiwan's opposition Kuomintang party.
Ms Chen arrived in China on Tuesday, becoming the first sitting KMT chairman to visit in over a decade.
She has consistently advocated for greater dialogue with Beijing.
Her party has repeatedly blocked attempts by Taiwan's ruling Democratic Progressive Party to advance its £32billion defence spending package.
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