Japan's Mainichi Shimbun newspaper said the meeting lasted about five minutes and that the two remained standing the entire time, apparently to emphasize its impromptu nature. The newspaper said the men reiterated their basic positions, albeit in a polite manner, and that Abe said bilateral relations should develop "on a strategic basis."
In Beijing, Foreign Ministry spokesman Hong Lei refused to say what had prompted the meeting or whether there had been a change in conditions between the countries. Hong instead reiterated Xi's reported remarks and said Japan, a close ally of China's main rival, the United States, needed to "gain the trust of the international community."
While short and informal, the exchange was the first the two have had since Abe returned as prime minister and Xi took over as head of the ruling Communist Party last year.
Chinese anger at Japan's move to buy the tiny, uninhabited islets in the East China Sea sparked violent protests and destruction of Japanese property in several Chinese cities. Beijing also began sending patrol ships to confront Japanese vessels in waters surrounding the islands, called Senkaku by Japan and Diaoyu by China.
While tensions have receded somewhat in recent months, China has yet to restore regular contacts and called in the Japanese ambassador last month to protest over visits by Japanese ministers to a Tokyo shrine at which fallen soldiers, including convicted war criminals, are worshipped.
Despite that, Japan remains a key trade partner and investor in China, while nearly 700,000 Chinese nationals are believed to reside in Japan.
Chinese leaders regard the relationship in practical terms as their second-most important foreign connection after that with the U.S., said Joseph Cheng, China politics expert at the City University of Hong Kong.
While China's official standoffishness, as seen in last week's Foreign Ministry statement, is mainly for domestic consumption, the country's leaders recognize the need to maintain contact, such as having the brief meeting in Russia, Cheng said.
"It's a nice gesture that can't be criticized at home, but which shows that China values the relationship," Cheng said.
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AP writer Yuri Kageyama in Tokyo contributed to this report.