PRESIDENT Barack Obama kicked off his four-nation Asia trip by becoming the first US leader to declare that the United States is obliged to defend Japan if disputed islands in the East China Sea are attacked.
"The Senkaku islands are administered by Japan and therefore fall within the scope of Article 5 of the US-Japan Treaty of Mutual Cooperation and Security," he said in a written interview with the leading Yomiuri Shimbun daily ahead of his trip.
"And we oppose any unilateral attempts to undermine Japan's administration of these islands," said Mr Obama, who arrived in Tokyo yesterday.
Article 5 spells out US defence obligations, requiring Washington to go to its ally's aid should the Senkaku islands - which the Chinese call Diaoyu and also claim - come under siege.
Chinese Foreign Ministry spokesman Qin Gang yesterday called on the US to respect its promises not to take sides in territorial disputes.
Mr Obama's message will no doubt please Prime Minister Shinzo Abe, who is seeking his help to fend off China. Besides making claims to the Senkaku islands and island groups in the South China Sea, Beijing has also unnerved countries in the region with its sudden announcement of an air defence zone last November.
But Mr Obama's explicit support for Japan does not necessarily suggest an improvement in the bilateral relationship. This has been less than warm since then premier Yukio Hatoyama in 2009 sought a "close and equal" Japan-US relationship, in effect asking for a more independent role for Japan. As for Mr Abe, his nationalistic stance did not make him immediately welcome in the White House after he took office in December 2012. The two men are not known to have cosy personal ties.
Mr Abe's failure to mend frayed ties with Japan's neighbours also means Mr Obama has a difficult balancing act to do.
Reports said that the joint statement to be issued after their talks today will reaffirm the two countries' security cooperation and pledge to play a leading role in ensuring peace and prosperity in Asia. But it will make no reference to the Senkaku dispute.
For while the US is willing to demonstrate its support for Japan, it does not wish to upset China, with which it is seeking to build strategic economic and political ties.
The fact that Washington's two closest allies in the region - Japan and South Korea - are barely talking to each other complicates Mr Obama's vision of a strategic "rebalancing" to Asia.
Part of the aim of his long-postponed Asia tour is to reassure Washington's friends of its continued commitment to the region and the US military and economic presence there.
"How he will reassure his country's allies without leaving the impression the United States and its allies are ganging up on China will be a test of US President Barack Obama's political acumen," said a commentary in the China Daily.
It is billed as the first official visit by a US president to Japan in 18 years, but the status of Mr Obama's trip does not reflect the degree of closeness in the bilateral relationship, observers say.
Rather, it reflects the insecurity of Mr Abe, who was said to have strongly requested that the visit be upgraded from an initial one-night stopover - and who used a private sushi dinner last night to bond with the US leader.