Xi, who is making his first foreign trip since the beginning of the pandemic, is attending a meeting this week of the China- and Russia-led Shanghai Cooperation Organisation in the Uzbek city of Samarkand.
However, he was absent from group photographs published late on Thursday when the leaders, including Russian President Vladimir Putin and Turkey's Tayyip Erdogan, went for dinner.
An Uzbek government source told Reuters Xi's absence and said the Chinese delegation cited its COVID-19 policy as the reason.
In Beijing, the Chinese foreign ministry did not immediately respond to a request for comment.
Xi, 69, is set to secure a historic third leadership term at a Communist Party congress that will begin next month.
Putin had on Thursday said he understood Xi had questions and concerns about Russia's war in Ukraine but praised China's leader for what he said was a "balanced" position on the conflict.
"During today's meeting, we will of course explain our position," Putin said.
"We will explain in detail our position on this issue, although we have talked about this before."
A Chinese readout of the meeting did not mention Ukraine. It said China was willing to give strong support to Russia for matters related to its core interests, state broadcaster CCTV reported.
China has refrained from condemning Russia's operation against Ukraine or calling it an "invasion", in line with the Kremlin, which casts the war as "a special military operation".
The last time Xi and Putin met in person, just weeks before Russia's February 24 invasion of its neighbour, they declared a "no limits" partnership and inked a promise to collaborate more against the West.
While Xi has now met Putin in person 39 times since becoming China's president in 2013, he has yet to meet Joe Biden in person since the latter became US President in 2021.
Xi's trip to Kazakhstan and Uzbekistan is his first outside China since the start of the COVID-19 pandemic.
His last trip outside China was a visit to Myanmar in January 2020.
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