Chinese President Xi Jinping and Russian President Vladimir Putin have been overheard discussing organ transplants as a means of prolonging life on the sidelines of a military parade in Beijing.

Putin suggested even eternal life could be achievable as a result of innovations in biotechnology, according to a translation of remarks caught on a hot mic.

The unguarded moment was captured on a livestream carried by Chinese state TV as the two leaders and North Korea's Kim Jong Un walked together through China's historic Tiananmen Square.

Xi and Putin have been in power for 13 and 25 years, respectively. Neither has expressed any intention of stepping down.

Standing shoulder-to-shoulder with the North Korean and Russian leaders, China's president used his Victory Day spectacle to project an alternate vision for the future of the world order.

However, their private conversation suggests their sights extend beyond the economic and political.

The exchange was relayed by a Mandarin translator for Putin and a Russian translator for Xi, and has been translated into English by the BBC.

"In the past, it used to be rare for someone to be older than 70, and these days they say that at 70 one's still a child," Xi's translator could be heard saying in Russian.

An inaudible passage from Putin follows. His Mandarin translator then added: "With the development of biotechnology, human organs can be continuously transplanted, and people can live younger and younger, and even achieve immortality."

Xi's translator then said: "Predictions are, this century, there's a chance of also living to 150 [years old]."

Putin reportedly reprised his remarks later while speaking to Russian media.

Russian state news agency Tass quoted him as saying: "Modern recovery methods, medical methods, even surgical ones dealing with the replacement of organs, enable humanity to hope for active life to last longer than it does today.

"Average age is different in different countries, but life expectancy will increase significantly".

Xi said the world faced a choice between peace and war as China unveiled a huge arsenal of weapons - including nuclear missiles with a global reach - to mark the 80th anniversary of the end of World War Two.

Wednesday's parade marked the first time that the Chinese, Russian and North Korean leaders had appeared together publicly, viewed by some observers as a message to the Western nations that have shunned them.

Putin and Kim joined 24 other dignitaries at the parade, including Iran's President Masoud Pezeshkian, Pakistan's PM Shehbaz Sharif, Vietnam's Luong Cuong and Zimbabwe's Emmerson Mnangagwa.

China has sought to position itself as a possible counterweight to the US since the imposition of Donald Trump's tariffs.

On Wednesday, the US president accused Xi of conspiring against the US with the leaders of Russia and North Korea.

In a post on Truth Social, Trump wrote: "Please give my warmest regards to Vladimir Putin and Kim Jong Un as you conspire against the United States of America."