"The policy of the United States is clear," he said in a written response to questions published in Japan's Yomiuri newspaper before his arrival in Tokyo at the start of a four-country Asia tour.
"The Senkaku islands are administered by Japan" and therefore fall under the U.S.-Japan treaty, he wrote. "And we oppose any unilateral attempts to undermine Japan's administration of these islands."
His statement seems aimed at reassuring Japan that the U.S. would come to its defense if China were to seize the islands, known as Diayou in China. Russia's annexation of Crimea has sparked concern about America's political will to protect Asian allies, notably in Japan and the Philippines.
A Chinese government spokesman responded that China has "indisputable sovereignty" over the islands, and said "the so-called Japan-U.S. alliance" should not harm China's territorial rights.
"We firmly oppose applying the Japan-U.S. security treaty in the issue of the Diaoyu islands," Foreign Ministry spokesman Qin Gang said at a regular briefing. "The U.S. should respect facts, take a responsible attitude, remain committed to not taking sides on territory and sovereignty issues, speak and act cautiously, and earnestly play a constructive role in regional peace and stability."
Obama told the Yomiuri the United States is deepening its ties with China, but "our engagement with China does not and will not come at the expense of Japan or any other ally."
He said the U.S. will continue to take steps to reduce the impact of its military presence in Okinawa, but added, "it's important to remember that the U.S. Marine Corps presence on Okinawa is absolutely critical to our mutual security. It plays a key role in the defense of Japan."