The US president has reiterated that islands at the centre of a Sino-Japanese dispute are covered by a US-Japan defence pact, in a public show of support for allies in Tokyo.
In comments made on Thursday in Japan, Barack Obama said that the islands - known as Senkaku in Japan and Diaoyu in China - were historically administered by Tokyo and came under the terms of joint-defence alliance.
He said US commitment to the treaty was "absolute". "This is not a new position. This is a consistent one," he said. "Article five covers all territories under Japan's administration including Senkaku islands."
However, he said: "We stand together in calling for disputes in the region, including maritime issues, to be resolved peacefully through dialogue."
He said the US commitment to defend Japan was a matter of historical fact rather than a rebuke to China: "The treaty preceded my birth, so obviously this isn't the red line that I'm drawing."
China later on Thursday said that the islands were its property.
During the same news conference, Shinzo Abe, Japan's prime minister, said that he and Obama had agreed to broaden their nation’s co-operation on defence issues, adding that Obama welcomed Tokyo's review its pacifist constitution.
Changing the constitution, which was agreed after defeat in the second world war, could leave Japan free to increase military spending, fight overseas, and conduct joint operations with its allies.
The row over ownership of the Senkakus has come to the fore in the last two years, with paramilitary vessels from both sides jostling in nearby waters to assert control.
In November, China declared an air defence identification zone over the East China Sea, including the skies above the islands. The US sent B52 bombers on patrol to test that zone, drawing criticism from Beijing.
Obama's comments came after an interview with the Yomiuri Shimbun newspaper on Wednesday, in which he said: "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.
"And we oppose any unilateral attempts to undermine Japan's administration of these islands."
Obama is on a week-long tour of Asia, part of what is being dubbed by the White House a "rebalancing" eastward of US foreign policy.
He will also discuss a new Pacific trade partnership in meetings with the leaders of the Philippines, Japan, Malaysia and South Korea. Although China is not on his itinerary, it is central to issues being discussed on every leg of the tour.