With tensions high in the disputed South China Sea, it would seem an odd moment for Beijing to pick another territorial fight with a neighbor.
Still, China chose to send more ships near Japanese-administered islets in the East China Sea in recent months, triggering a flurry of protests from Tokyo. That's even as President Xi Jinping prepares to host global leaders including Japan's Prime Minister Shinzo Abe in Hangzhou this weekend.
The strains come amid other frictions in North Asia, including Chinese criticism of South Korean President Park Geun-hye's plan to deploy a U.S. missile shield in her country. That points to a potentially chilly Group of 20 summit and clouds the prospects for any Xi-Abe or Xi-Park sit down.
One of the reasons China may be escalating its activity in the East China Sea is to warn Japan against getting involved in the South China Sea spat. While not a claimant in those waters, Japan has drawn China's ire for supporting Southeast Asian nations, through means such as providing patrol boats to the Philippines.
"China is certainly ratcheting up tensions over the Senkaku/Diaoyu islands," said Chris Hughes, professor of international politics and Japanese studies at the University of Warwick in the U.K., using the Japanese and Chinese names for the East China Sea islets.
"There clearly is a domestic political need in China to demonstrate strength on territorial issues, especially after the Permanent Court of Arbitration's ruling in July, and this applies equally to Japan in the East China Sea as well as to the South China Sea," he said.
The arbitration court in the Hague invalidated most of China's claims in the South China Sea in its decision last month. Japan has repeatedly urged all parties to abide by the ruling.
On Aug. 23, Chinese Foreign Ministry spokesman Lu Kang said Japan had "taken a series of negative moves" in terms of its relationship with China, "especially when it keeps hyping up the South China Sea issue."
There is a "possibility" that Japan may raise the territorial issues at the G-20 "to stir trouble," according to a commentary published Tuesday in China's state-run Global Times. It urged Japan to "act in tune with the theme of the summit instead of causing trouble."
The latest tensions have not dented Chinese tourism to Japan — the 731,400 visitors in July were a monthly record — or impacted trade. But they further complicate ties just as Asia's two biggest economies face headwinds, as well as adding to the risk of a physical clash.
Sino-Japanese ties hit a particularly icy patch in 2012 when Japan nationalized three of the East China Sea islands, and China announced an air defense identification zone over the waters in late 2013. The first time Abe and Xi met formally, at the Asia-Pacific Economic Cooperation forum in Beijing in late 2014, it proved a brief and frosty affair.
Since then, however, tensions over the area had receded somewhat as China focused on asserting its claims in the South China Sea, through which some of the world's busiest shipping lanes run.
China's actions also represent a fresh effort to gain the advantage in the East China Sea, according to Jia Qingguo, a member of the foreign affairs standing committee of China's top political advisory body.
"Previously, the pattern was they took action and we made a response; they threw a problem at us and we had a crack at it," said Jia, who is also a professor at Peking University. "Now China is seeking to take the initiative and seize the upper hand. We make an opening gambit and they try to respond," he said.
"They haven't got used to the new situation yet, but there is not much they can do but gradually adapt to it."
After a meeting with his Japanese and South Korean counterparts last Wednesday in Tokyo, Chinese Foreign Minister Wang Yi said China is considering Japan's request for a summit on the G-20 sidelines, but warned the countries needed a "good atmosphere" for that to occur. China values Abe's "positive attitude" and the Japanese government's "support" for the Hangzhou G-20, Wang said.
Japan repeatedly protested the almost daily incursions by Chinese vessels into what it considers its territorial waters. Its Coast Guard released video footage showing more than 200 Chinese fishing boats were accompanied by 28 government ships in the area during the August 5-9 period — the largest group since September 2012.
On August 18, six Chinese naval ships conducted live-fire drills in the Sea of Japan. The countries have yet to put into force a maritime and aerial communications system to prevent unintended clashes, although it has been under discussion for some time.
"At every step they take in the elevation ladder, Japanese frustration and anxiety grows," said Alessio Patalano, a senior lecturer in war studies at King's College London. "It is undeniable that mutual perceptions are deteriorating and the risk of unintended consequences continues to mount."
As it faces off against China, Japan plans to develop a longer range land-based anti-ship missile that could reach the edge of the disputed islands, with the Defense Ministry seeking funding in a budget request to be submitted soon, according to the Yomiuri newspaper.
While Abe has appeared keen to meet again with Xi, there is a limit to what he can tolerate, given the need to avoid appearing weak at home. He is riding high in opinion polls after an appearance dressed as Super Mario at the Rio Olympics closing ceremony, but a poll this week showed a majority of the public want a stronger stance on China.
While the foreign ministers were talking in Tokyo, Shotaro Yachi, Japan's National Security Council chief and an aide to Abe, was in Beijing, suggesting the door to a summit is open. Yachi handed Premier Li Keqiang a letter from Abe expressing a desire to stabilize ties, according to China's foreign ministry, while Li urged Japan to use the coming year -- the 45th anniversary of the normalization of ties -- to improve relations.
Though Chinese activity in the East China Sea has leveled off in recent days, Xi has reason to continue to want to appear strong at home: Next year will see a mid-term power transition where the majority of the ruling Politburo and its supreme Standing Committee are set to be replaced. Xi's term runs for a further five years.
The current situation is "a combination of Japan's less apologetic approach and the sensitive political situation during the Chinese leadership transition," said Tsuneo Watanabe, a senior fellow at the Tokyo Foundation.
"It won't be easy to create a reconciliatory atmosphere until the Chinese leadership is consolidated after summer 2017."