Chinese President Xi Jinping joked about security backdoors while presenting a pair of Xiaomi Corp. smartphones to his South Korean counterpart, a rare moment of spontaneous levity captured during a week of tense trade negotiations with Donald Trump.

Xi, in South Korea to meet Trump on the sidelines of the Asia-Pacific Economic Cooperation summit, presented the pair of devices to Korean President Lee Jae Myung. In a video circulated on social media, Lee asked: “Is the line secure?” Xi chuckled, pointed at the gadgets and replied through an interpreter: “You can check if there’s a backdoor.” The two leaders burst into laughter.

The exchange was striking because the issue of security and alleged espionage is a sensitive one and a major thorn in U.S.-Chinese relations. American lawmak

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