An American warplane was monitored by China’s military as it flew over northern Taiwan this week, according to a PLA source, despite the United States denying – after first acknowledging – that any such flight ever took place. The incident began when two flight tracking services – Golf9 and Tokyo Radar – reported on Twitter on Wednesday that an aircraft with the serial number 62-4134 had flown through Taipei’s airspace at an altitude of 31,500 feet. Although Taiwan’s air force dismissed the reports as fake news, the US Air Force on Wednesday confirmed that one of its RC-135W electronic surveillance planes had been in the area at the time of the reported sighting. “I can confirm that an RC-135W aircraft did fly over the northern portion of Taiwan yesterday as part of a routine mission,” the US Pacific Air Forces’ public affairs department said in a statement published by military news website The War Zone. “Due to operational security, we are not able to discuss the specifics of the mission,” it said. But on Friday it did an about-face, with one of its senior officers denying any knowledge of the flight. “I would like to correct the record by stating we did not have any US aircraft in that area on the date and time in question,” Lieutenant Colonel Tony Wickman, director of public affairs for Pacific Air Forces, was quoted as saying by The War Zone. Despite the US air force’s apparent confusion, and Taiwan’s denial, a source from inside the People’s Liberation Army – the Chinese mainland’s military – said the flight did take place and was closely monitored. “China tracked the whole flight path of the US warplane over Taipei on Wednesday to ensure the operation did not have a malicious intent,” the person told the South China Morning Post on Saturday. How Beijing’s ‘red lines’ over Taiwan could lead to war with US While the PLA did not respond to the aircraft’s transit, the source said the US operation was risky and could have been misinterpreted. “Similar operations could easily make China miscalculate US intentions and it’s highly unprofessional for a military plane to fly over a populous city … it could trigger unnecessary trouble and conflicts,” he said. An image provided by Tokyo Radar showed the aircraft flew from north to south over Taipei, Taiwan’s capital and home to about 2.6 million people. Collin Koh, a research fellow at the S Rajaratnam School of International Studies at Singapore’s Nanyang Technological University, said it was unlikely the US aircraft had mistakenly flown over Taipei. “The aircraft is one of the most sophisticated reconnaissance platforms in the world,” he said. “There’s no way it has a mediocre navigation system, and there’s no way its crew would make such an error.” US warplane flies along dividing line between mainland China and Taiwan A more likely explanation was that the two air forces – after reports of the flight had entered the public domain – sought to play down the matter so as to avoid stirring up more trouble in the region, Koh said. “Although these flights are classified … their movements can still be tracked by commercially available air traffic information systems online,” he said. “The Taiwanese responded to reports on this mission and it appears they denied it because they didn’t want to inflame the already tense Taiwan Strait situation .” Hu Xijin, editor-in-chief of the nationalistic Chinese tabloid Global Times , said that whatever the circumstances surrounding the flight, Beijing would have been closely monitoring any activity close to its shores. “If US warplanes appear in the island’s [Taiwan’s]airspace, it’s reasonable for PLA warplanes to expel them and defend China’s sovereign rights,” he said on Weibo, China’s Twitter-like platform on Saturday. Beijing regards self-ruled Taiwan as part of its territory awaiting reunification with the mainland. After Washington this week approved the sale of long-range cruise missiles to Taiwan , China’s foreign ministry accused it of “seriously violating the one-China principle and the three communiques” that underline US-China relations .