US President Barack Obama has angered Beijing by saying the uninhabited Diaoyu Islands in the East China Sea, which are claimed by China and Japan, would be covered by the US-Japan security treaty. He is the first US president to spell this out.
He gave the assurance as he began a four-nation Asia tour in Tokyo yesterday.
Watch: Imperial pomp starts Obama's Japan visit
China was quick to hit back. "The so-called security alliance between the US and Japan is a bilateral arrangement made during the cold war period, and it should not be used to damage China's sovereignty and legitimate interest," foreign ministry spokesman Qin Gang said. "We resolutely oppose applying the Diaoyu Islands to the Japan-US security treaty."
China and Japan are engaged in a bitter territorial dispute over the Diaoyus, known as Senkakus in Japan. The risks of an unintended conflict were highlighted in January when Tokyo alleged that a Chinese frigate locked its radar on a Japanese destroyer in the East China Sea. In December, the US military cruiser Cowpens had to take evasive action in the South China Sea to avoid hitting a warship supporting China's first aircraft carrier, Liaoning.
Adding fuel to those fears, both PLA Navy commander Admiral Wu Shengli and Japanese Maritime Self-Defence Force chief of staff Katsutoshi Kawano, attending a defence forum in Qingdao this week, said an accidental conflict between the two militaries could not be ruled out, according to Phoenix TV reports.
In written remarks published by Japan's Yomiuri Shimbum, Obama said the US alliance with Japan was "stronger than ever".
"The policy of the United States is clear - the Senkaku Islands are administered by Japan and therefore fall within the scope of Article 5 of the US-Japan Treaty of Mutual Co-operation and Security," Obama said. "And we oppose any unilateral attempts to undermine Japan's administration of these islands."
Article 5 of the treaty, which was signed in 1960, stipulates US defence obligations to Japan, stating that each party recognises that an armed attack against either party in the territories under the administration of Japan would be dangerous to its own peace and safety.
Obama said he had told President Xi Jinping that the territorial disputes should be resolved through dialogue and diplomacy, and also commended Japanese Prime Minister Shinzo Abe's efforts to strengthen Japan's defence forces.
"We believe that it's in the interest of both our countries for Japanese Self Defence Forces to do more within the framework of our alliances," he said.
Obama's tour, which will also take him to South Korea, the Philippines and Malaysia, came as Asian nations fear that US commitment to the region will be lessened, particularly as Obama cancelled his tour last year because of financial problem at home.
In signs that China was keeping a close watch on the tour, Xi had a phone conversation with South Korean President Park Geun-hye, saying bilateral ties enjoyed a "sound momentum", state-run Xinhua reported.
Xi also told Park that China was committed to the denuclearisation of the Korean Peninsula.
Shi Yinhong , a US affairs expert at Renmin University, said Obama made the unprecedented assurance because Washington needed Tokyo's support for the Trans Pacific Partnership, a US-dominated trade pact.
Obama to set aside decades of uneasy ties with Malaysia visit
Barack Obama will this weekend become the first US president in nearly 50 years to visit Malaysia, where he will seek to put decades of uneasy bilateral relations behind him as both nations cast wary eyes on a rising China.
Mindful of America's image problem in the Islamic world, Obama is expected to tout the US friendship with the thriving moderate Muslim nation.
Malaysia is also an important partner in the US "rebalance" of its strategic attention to Asia, where concern is rising over China's territorial assertiveness.
Obama will "highlight the growing strategic and economic relationship" with Malaysia and its "credentials as a moderate, Muslim-majority state and emerging democracy", said Joshua Kurlantzick, a fellow at the US Council on Foreign Relations.
Prime Minister Najib Razak, meanwhile, will seek to capitalise on Obama's expected praise to counter flagging voter support and global criticism over the handling of the loss of Malaysia Airlines flight MH370.
Obama was five years old the last time a serving US leader visited Malaysia. Lyndon Johnson went in 1966 to rally support for the US war in Vietnam.
Tension followed during the 1981-2003 tenure of authoritarian leader Mahathir Mohamad, a harsh critic of US policies.
But ties - especially trade - remained solid, and the more Western-oriented Najib has sought even closer relations.
US deputy national security adviser Ben Rhodes this week expressed hope that the visit would "elevate US-Malaysian relations to a new stage".
Obama postponed a visit last year to deal with the US government shutdown.
Underlining the need for a reintroduction after nearly a half-century, Malaysia is the only stop on Obama's Asian swing to include a "town-hall meeting". He also visits close allies Japan, South Korea and the Philippines.
China will loom large. It is Malaysia's top trade partner and Najib has played down their rival maritime claims. But Malaysian anxieties have grown, particularly after China held naval exercises in disputed waters last year, and the US and Malaysia have moved recently to improve defence ties.
Chinese criticism over MH370 has also left a bitter aftertaste.
"[Najib's government] obviously hopes that Obama's star effect can rub off on its flagging popularity," said Dr Oh Ei Sun, senior fellow at the S. Rajaratnam School of International Studies in Singapore.
"And Malaysia can continue to counterbalance China with the US in its foreign policy - siding with China economically but with the US on security."
Differences remain, though.
The economic component of Obama's "rebalance" rests largely on his vision of a Trans-Pacific Partnership (TPP), a region-wide trade liberalisation pact bedevilled by rocky negotiations with partners. Malaysia has resisted free-market reforms that clash with its controversial policies reserving economic advantages for majority ethnic Malays.
Obama may also face pressure to address uncomfortable human rights, democracy and religion issues in Malaysia.