Angst at Japanese airlines
Japan Airlines and ANA Holdings, while complying with the government directive, are uneasy about flying through the zone without giving notice to China, especially after Washington advised U.S. carriers to comply, two sources familiar with the Japanese carriers' thinking told Reuters.
White House spokesman Jay Carney told a briefing that for safety reasons, U.S. carriers operated in accordance with notices issued by foreign countries.
"However - and let me be clear - this in no way indicates U.S. government acceptance of China's requirement in the newly declared ADIZ, and has absolutely no bearing on the firm and consistent U.S. government position that we do not accept the legitimacy of China's requirements," Carney said.
(Read more: Defying China, US bombers fly into East China Sea zone)
Washington takes no position on the sovereignty of the disputed islands, known as the Senkaku in Japan and the Diaoyu in China. However, it recognizes Tokyo's administrative control and says the U.S.-Japan security pact applies to them.
U.S., Japanese and South Korean military aircraft all breached the zone last week without informing Beijing and China later scrambled fighters into the area.
Biden dismissed doubts in Japan and elsewhere in the region over whether the United States has the resources to carry out a strategic "rebalance" that Washington says is a cornerstone of its foreign policy in Asia, given U.S. fiscal woes, its attention on the Middle East, and partisan battles at home.
Some experts said those doubts may have encouraged China to think the United States would not react strongly to its announcement of the air defense zone.
"Japan knows that we have stayed for more than 60 years, providing the security that made possible the region's economic miracle," he said.
(Read more: Airlines to inform China of flights over disputed area)
"Economically, diplomatically, militarily, we have been, we are, and we will remain a resident Pacific power."