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BEIJING, Sept 3 (Reuters) - When Russian President Vladimir Putin walked shoulder to shoulder with China's President Xi Jinping on Wednesday, a hot mic caught them discussing organ transplants and the possibility that humans could live to 150 years old.

The moment came as Putin and Xi walked with North Korea's Kim Jong Un at the head of a delegation of more than two dozen foreign leaders to view a military parade in Beijing marking the 80th anniversary of the end of World War Two.

The moment was carried on the livestream provided by state broadcaster CCTV to other media, including China-state broadcaster CGTN, AP and Reuters. China's radio and TV administration said CCTV's coverage of the event was viewed 1.9 billion times online and by more than 400 million on TV.

As Putin and Xi walked toward the Tiananmen rostrum where they viewed the parade with Kim, Putin's translator could be heard saying in Chinese, "Biotechnology is continuously developing."

The translator added, after an inaudible passage, "human organs can be continuously transplanted. The longer you live, the younger you become, and (you can) even achieve immortality."

In response, Xi, who was off camera, can be heard responding in Chinese, "Some predict that in this century humans may live to 150 years old."

Kim was smiling and looking in the direction of Putin and Xi, but it was not clear if the conversation was being translated for him. Putin cannot be heard speaking clearly in Russian in the CCTV clip.

The Russian government did not immediately respond to a request for comment. China's Ministry of Foreign Affairs and CCTV did not immediately respond to a request from Reuters for comment.

As Xi began speaking, the video cut to a wide shot of Tiananmen Square and the audio faded.

Just over 30 seconds later, Xi, Putin and Kim reappeared on camera as they walked on the steps toward the viewing platform for the parade.

At the event, Xi told a crowd of more than 50,000 spectators the world faced a choice of "peace or war" as he inspected troops and cutting-edge military equipment, including hypersonic missiles and naval drones. Putin arrived in China on Sunday to attend a summit organized by Beijing that convened more than 20 leaders of non-Western countries, including Indian Prime Minister Narendra Modi.

Putin and Xi signed more than 20 agreements, ranging from energy to artificial intelligence, and agreed to build a major new gas pipeline, without announcing key details on financing or the pricing of the gas that would be sent to China.