In September, there was an increase in the activity of refueling aircraft from the US Guam Air Force Base, which was used to fill the tanks of the surveillance aircraft. The majority of the aircraft appear to be flying over the South China Sea – although 13 aircraft have flown over the Yellow Sea and about six aircraft over the East China Sea.

The data was reported by the South China Sea Strategic Status Examination Initiative this week.

She said the increase in refueling activity in particular could indicate that the United States is preparing for more long-haul flights.

The research center said the activity was “unusual” given that refueling vehicles were dispatched from Guam Air Force Base.

It was easiest to take off from the US Kadena Air Force base in neighboring Japan.

She added: “Such operations are likely preparing for long-distance refueling in the future in extreme conditions, and therefore deserve great attention.”

The center said the activity showed the South China Sea was the “primary focus” of the United States.

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In one case, a US aircraft allegedly changed its code so that it appeared to be an aircraft coming from the Philippines before changing it again afterwards.

Once again, an RC-135S spy plane pretended to be a Malaysian plane while on a mission near Chinese airspace, the research center said.

In some cases, American aircraft did not operate their transceivers at all.

The SCSPI claimed to have confirmed the use of fake electronic identification codes by US military aircraft last month.

The think tank said the move would be of “significant practical value” for the US military because reconnaissance aircraft resemble civilian aircraft anyway.

However, she added: “This behavior undoubtedly adds significant risks and unstable factors to global aviation safety, which will cause errors in judgment and is likely to bring danger to real civilian passenger aircraft, particularly those from impersonation countries.”

On September 1, 1983, 269 civilian passengers and crew of a Korean airliner were killed after it was shot down by a Soviet Sukhoi SU-15 fighter jet.

The center said a Boeing 747 passenger plane was flying over the Soviet Sakhalin Island at the time, but was mistaken for a US RC-135 reconnaissance plane.

The South China Sea has been a point of tension between the United States and China throughout the year.

In addition to the two countries conducting military exercises in the region, the United States is conducting official contacts with nearby Taiwan, which angered Beijing.