Beijing, China - Chinese President Xi Jinping and Russian President Vladimir Putin were caught discussing immortality and organ transplants prior to Beijing's massive military parade, in a bizarre hot mic moment.

Xi and Putin discussed life-prolonging organ transplants and immortality as they walked and chatted together before Wednesday's parade, in comments picked up by state media microphones.

Historic images showed Xi shaking hands and speaking with Putin and North Korean leader Kim Jong-un as they walked down a red carpet by Tiananmen Square.

"These days... 70 years old," Xi told Putin in Mandarin, footage from state broadcaster CCTV showed, as the Chinese leader's translator conveyed his remarks.

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"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," the translator then told Putin, quoting a Tang dynasty poem.

Putin then turned towards Xi and the translator relayed Putin's remarks back to the Chinese President.

"With the development of biotechnology, human organs can be continuously transplanted," Putin reportedly said. "People could get younger as they grow older, and may even become immortal."

"Predictions are, in this century, it may be... possible to live to 150 years old," Xi responded.

When asked about the exchange by a reporter during a press conference after the parade, Putin didn't shy away from what they had talked about.

"Ah, I think it was when we were going to the parade that the Chairman spoke about this," he told reporters, referring to Xi.

"Modern means – both health improvement and medical means... allow humanity to hope that active life will continue not as it does today," Putin said.