US President Barack Obama's promise of military cover for Japan's claim on the Diaoyu Islands faces the potential of backfiring, observers said.
Obama stated in a press conference in Tokyo on Thursday that the US-Japan mutual security treaty covers China's Diaoyu Islands.
"We do not believe that they should be subject to change unilaterally, and what is a consistent part of the alliance is that the treaty covers all territories administered by Japan."
The forthright remarks from Obama are widely interpreted as a display of Washington's strong commitment to its Asian allies designed to dispel suspicion of weakening US clout in the region.
Obama is on a four-nation tour that was postponed seven months ago because of the US government shutdown. He faced flak at the time for postponing the trip, both in the US and overseas, amid criticism that the US was preoccupied with domestic affairs at the expense of its international commitments.
Responding to Obama's comments, Defence Ministry spokesman Yang Yujun said that the Chinese army will continue military patrols in "relevant waters" in the East China Sea.
The Chinese military is "fully capable of safeguarding the Diaoyu Islands, and it is unnecessary for other nations to go to extreme lengths to provide a so-called security guarantee," Yang said, adding that China will firmly safeguard territorial sovereignty in the face of provocation from Japan.
Ruan Zongze, vice-president of the China Institute of International Studies, said Obama's remarks may lead to unforeseen problems because the military commitment - directly naming specific islands - could "sabotage US strategic initiatives in the region" and undermine its strategic flexibility.
"As a result, Tokyo is keeping Washington in check in this regard, and, honestly, the ruling Japanese cabinet is very unpredictable," Ruan said.
Ruan noted that Obama's remarks about the islands "also harm the credibility of the US", because instead of taking an honest broker's viewpoint the US is firmly backing one side and this has the potential to cause problems.
With Japanese Prime Minister Shinzo Abe beside him, Obama told reporters that he had not drawn any new "red line" over the islands, and he emphasised the need to resolve maritime disputes peacefully.
"The treaty between the US and Japan preceded my birth, so, obviously, this isn't a red line that I'm drawing," Obama said.
Li Haidong, a researcher of US studies at China Foreign Affairs University, said Obama's visit to Japan aimed to boost Japan's status as a "pillar" of Asia-Pacific security and as a key player in containing China.
But, Li said, the two allies have different agendas.
"The US seeks stability in the big picture of its relationship with China, yet Japan is not afraid of fanning the flames of a conflict with China," Li said.
The US-Japan defence treaty requires Washington to come to Japan's defence if it is attacked.
Experts said Washington believes that backing Tokyo will have long-term benefits.
Lyu Yaodong, an expert on Japanese diplomacy at the Chinese Academy of Social Sciences, said Obama is "determined to see tangible progress in his rebalancing strategy" during his Asian trip, and "to achieve this goal, satisfying demands from Japan regarding the islands is necessary".
Abe told reporters on Thursday that "the Japan-US alliance is more robust than ever before."
"The US pivot cannot succeed without strong support from important allies such as Japan," Lyu said.
Ruan noted a shift in the US-Japan military relationship, and one example is that the US is "outsourcing" more defence duties to Japan.
"Washington believes that its promise on the islands is a feasible way to strengthen its influence over Japan, and accordingly Washington wants Japan's self-defence forces to play a greater role, which is very dangerous," Ruan said.
Yang, the Defence Ministry spokesman, also confirmed what Chinese Navy Commander Wu Shengli said on Wednesday about a worst-case scenario.
Wu told reporters on the sidelines of the Western Pacific Naval Symposium that the possibility of a military conflict remains between China and Japan, and the priority is to "prevent the outbreak of a conflict".
Yang also said the PLA will continue military patrols in waters near a tropical Japanese island close to Taiwan, days after Tokyo announced it would break ground on a new radar base in the area.
The radar station on Yonaguni Island, just 150 km from the Diaoyu Islands, marks Japan's first military expansion at the western end of its island chain in more than 40 years.
"We are paying close attention to Japan's military trends," he said.
"China's military will continue to carry out battle readiness patrols, military drills and other activities in the area," Yang said.