"We have contingency plans and we discuss them with allies," a US official told AFP speaking on condition of anonymity, saying it was "natural" that the two governments would confer on emergency scenarios given recent tensions.
A Pentagon official, who also asked not to be quoted by name, confirmed the discussions, saying "we're a planning organization."
But both sources said the US government did not want to fuel tensions, and that the contingency planning would be only one of many topics on the agenda when top US and Japanese officers meet in Hawaii later this week.
Admiral Samuel Locklear, head of US Pacific Command, is scheduled to host General Shigeru Iwasaki, chief of the Japanese Self Defense Forces Joint Staff, for Thursday's talks.
Officially, the Pentagon would neither confirm nor deny whether the contingency plans were under discussion.
"As a matter of policy, we do not discuss our military planning efforts," said Lieutenant Colonel Catherine Wilkinson.
"The US policy on the Senkaku Islands is long-standing. We encourage the claimants to resolve the issue through peaceful means," she said, using the Japanese name for the islands.
The United States has made clear that its alliance with Tokyo applies to the islands, raising the possibility of US military action in support of Japan if China moves to seize them.
Beijing and Tokyo both claim the islands, which the Chinese call the Diaoyu.
The dispute has escalated in recent months, with Beijing repeatedly sending ships to waters around the islands to back up its claims. Tokyo has alleged that a Chinese frigate locked its radar on a Japanese destroyer in January.
In a faxed response to a query, China's defense ministry said it had seen the Nikkei report and reiterated Beijing's stance that the islands belong to China.
"The determination and will of Chinese military forces to safeguard China's sovereignty and territorial integrity are steadfast," the ministry said.
"We firmly oppose any action that could further complicate and magnify the situation."
China's newly installed President Xi Jinping is vowing to fight for a "great renaissance of the Chinese nation." Xi has close ties to China's expanding military, and called for the armed forces to strengthen their ability to "win battles."
Japan too has expressed a new strain of nationalistic rhetoric under its hawkish prime minister, Shinzo Abe, who has urged new graduates of the National Defense Academy to guard the country against "provocations."
Abe's Liberal Democratic Party pledged at its annual convention on Sunday to accelerate efforts to reform Japan's pacifist post-war constitution and create a fully fledged military.