The lighthearted exchange took place on Saturday in the city of Gyeongju, when Xi and President Lee Jae Myung presented gifts to each other on the sidelines of an APEC summit, marking Xi's first visit to South Korea in more than a decade.

Xi presented two Xiaomi smartphones fitted with Korean-made displays to Lee, who quipped: "Is the communication line secure?", drawing laughter from Xi.

Pointing at the devices, Xi replied: "You should check if there is a backdoor," referring to pre-installed software that could allow third-party monitoring, prompting laughter and applause from Lee.

The brief banter sparked heavy media interest over the weekend, as Xi is rarely seen making jokes, let alone about espionage.

"Xi bursts into laughter after Lee jokes about security of Xiaomi Phones," reads a headline in the Seoul Shinmun daily on Monday.

One video of the exchange on YouTube attracted more than 800 comments, many expressing surprise at the exchange.

"It feels like martial arts masters trading lines in a duel," wrote one user with the handle 021835.

The moment of levity underscored how the two leaders had grown closer during a series of encounters over two days, Lee's spokesman Kim Nam-jun told AFP.

"From welcoming ceremonies and gift exchanges to a banquet and cultural performances, both leaders had multiple opportunities to engage and build personal chemistry," he said.

"If it weren't for such chemistry, that kind of joke would not have been possible."

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