Regular patrols by Japanese aircraft only reach into the East China Sea, where Japan is at loggerheads with China over disputed islands. Extending surveillance flights into the South China Sea would almost certainly increase tension between the world's second- and third-largest economies.
"I think allies, partners and friends in the region will look to the Japanese more and more as a stabilizing function," Admiral Robert Thomas, commander of the Seventh Fleet and the top U.S. navy officer in Asia, said in an interview.
"In the South China Sea, frankly, the Chinese fishing fleet, the Chinese coastguard and the (navy) overmatch their neighbors," Thomas said.
China's foreign ministry said it had no immediate comment on the interview.
Thomas's comments show Pentagon support for a key element of Japan Prime Minister Shinzo Abe's push for a more active military role in the region. That is crucial because U.S and Japanese officials are negotiating new bilateral security guidelines expected to give Japan a bigger role in the alliance, 70 years after the end of World War Two.
"I think that JSDF (Japan Maritime Self Defense Forces) operations in the South China Sea makes sense in the future," Thomas said.
Japan is not party to the dispute in the South China Sea where China, the Philippines, Vietnam, Taiwan, Malaysia and Brunei have competing claims. But the waterway provides 10 per cent of the global fisheries catch and ships crossing it carry $5 trillion a year in cargo, a large portion to and from Japan.
A Japanese Defence Ministry spokesman was not available for comment. Japan has never indicated any intention to patrol the South China Sea.
The ministry routinely declines to comment on any changes to guidelines being discussed with the United States. It calls for peaceful solutions to disputes and for all countries to follow international law and conventions.
"MORE ROBUST AND PROACTIVE"
Abe is pushing for legislation later this year that would allow Japan's military to operate more freely overseas as part of a broader interpretation of the self-defense allowed by its pacifist constitution.
Those changes coincide with the deployment of a new Japanese maritime patrol plane, the P-1, with a range of 8,000 km (5,000 miles). That is double the range of current aircraft and could allow Japan to push surveillance deep into the South China Sea.
"This is a logical outgrowth of Abe's push for a more robust and proactive military. It is also a substantial departure from JSDF's customary operations," said Grant Newsham, a research fellow at the Japan Forum for Strategic Studies and a former U.S. Marine liaison officer to Japan's military.
Newsham said sending surveillance aircraft to the South China Sea would allow Japan to deepen its military ties with countries like the Philippines, one of Abe's goals to counter China's growing naval power.
China has outlined the scope of its claims with reference to a so-called nine-dash line on its maps that takes in about 90 percent of the South China Sea.
"The alleged nine-dash line, which doesn’t comport with international rules and norms, standards, laws, creates a situation down there, which is unnecessary friction," said Thomas.
The Scarborough Shoal near the Philippines is one flashpoint in the South China Sea. The Philippines complains that China has kept its fishermen from waters around the shoal. Thomas said Japan could help the Philippines with equipment and training.
"For the Philippines, the issue is one of capacity. For the Japanese that is a perfect niche for them to help, not just in equipment, but in training and operations," he said.
Philippine Defense Secretary Voltaire Tuvera Gazmin met Japanese Defense Minister Gen Nakatani in Tokyo on Thursday, and they signed a memorandum on cooperation that includes military exchanges, training and defense technology and equipment.
Gazim told Nakatani that "common security concerns provide an opportunity to deepen defense cooperation".
Nakatani said the agreement had "taken defense ties to a new level". He declined to say if China's presence in the South China Sea and East China Sea had been discussed.
Centered around the USS George Washington carrier battle group with its home port in Japan, the U.S. Seventh Fleet includes some 80 vessels, 140 aircraft and 40,000 sailors making it the most powerful naval force in the western Pacific.
Japan Maritime Self-Defense Force Kawasaki P-1 maritime patrol aircraft intended to replace the P-3C Orion.