Although White House officials stressed that the state visit by Japanese Prime Minister Shinzo Abe is about the US-Japan partnership, China was the nation that they mentioned most frequently during their joint press conference in White House on Tuesday.

Calling Abe a "partner and friend", Obama assured him of the US commitment to Japan's security by announcing that the two countries have updated their defense guidelines for the first time in two decades.

"I want to reiterate that our treaty commitment to Japan's security is absolute," said Obama, with Abe standing nearby in the White House Rose Garden.

Obama said the enhanced defense commitment would cover "all territories under Japan's administration, including the Senkaku Islands"(called Diaoyu Islands in China), a group of uninhabited small islands in the East China Sea that was Chinese land since ancient times.

Observers said the Obama administration's latest policy agenda shows that the US views China's rising strategic influence in the region as an imminent challenge and that Tokyo has secured more tangible support from Washington during Abe's visit.

The Abe administration's earlier effort to "nationalize" the Diaoyu Islands has soured Sino-Japanese relations.

Under these guidelines, announced on Monday and revised for the first time since 1997, Japan will have the right to exercise collective self-defense – being allowed to defend not just its own territory, but also the United States and other countries if needed.

"By so doing, the deterrence would be enhanced," Abe said. "The Japan-US alliance would be more efficient and more functional. Deterrence and response capabilities would be heightened as a result," said the first Japanese prime minister to visit the US since 2006.

Chinese Foreign Ministry spokesman Hong Lei said on Tuesday, "Both the US and Japan have a duty to ensure that their alliance does not infringe the interests of third parties, including China, or the peace and stability of the Asia-Pacific region."

China's territorial sovereignty over the Diaoyu Islands and affiliated islets "remains intact no matter who makes remarks or (takes) action against them", he said. He also said the way in which the US-Japan treaty alliance, formed during the Cold War, develops "deserves attention from all parties".

Teng Jianqun, an expert at the China Institute of International Studies, said the Obama remarks mirrors a growing priority over the alliance with Japan, and the remarks about China's allegedly flexing muscles "aim to impress the public that the US gives a strong back to Abe and Japan". Teng also believes that "it is unlikely for Tokyo to shirk away from the Comfort Women issue".

Feng Wei, an expert on Japanese studies at Fudan University in Shanghai, said the increasing investment by Washington and Tokyo over their alliance "partly aims at countering China", and it is also because of Washington's shrinking influence upon the NATO. "In this context, Japan is expected to take a bigger role in Washington's global strategy," Feng said.

Yang Bojiang, deputy director of the Institute of Japan Studies at the Chinese Academy of Social Sciences, said Washington is working on championing its leadership over the region through the evolving US-Japan agenda, and Japan seems to gain weight within the traditional alliance.

"The US is trying to tie Japan tightly to its future strategies," Yang said, adding that the US "cannot accept that its leading role in the region is challenged".

Japan's move toward a more aggressive military approach in recent years, including Abe's efforts to seek a new interpretation of Japan's antiwar constitution, imposed during the American postwar occupation, has worried neighboring countries, including China and South Korea.

Responding to a question that labels the guidelines as more "assertive", Obama said it should not be seen as a provocation of Beijing.

Ruan Zongze, vice-president of the China Institute of International Studies, said the US support of Japan increasing its military capacity through the alliance would further distance other Asian countries from Japan and would be a concern to China, especially when Japan's posture on historical and territorial issues have hampered the relations.

During the press conference, Obama also declared the progress the two have made on the 12-member Trans-Pacific Partnership (TPP) – in which the US and Japan are the two largest economies – as the major achievement of the visit.

Obama said he and Abe have decided the two countries would work together to bring a "swift and successful conclusion" for the TPP, calling it "the most progressive trade bill in history."

Obama said the TPP is good for American workers "regardless of what China is doing". In an earlier interview with The Wall Street Journal, Obama argued that failing to complete the TPP deal would increase China's influence in the Asia Pacific.

US and Japanese negotiators have yet to work out the final details of a US-Japan trade deal, which is essential to the TPP. Obama and Abe also both face domestic resistance to the deal.

Obama dispelled the notion that the US is opposed to other countries participating in the Asia Infrastructure Investment Bank (AIIB), which China proposed and of which it would serve as a main donor. Abe said Japan was undecided, but he is cooperating with the US on the issue, and said he intends to continue dialogue with China.

"China's latest strategic proposals of regional influences – including the Asian Infrastructure Investment Bank – have indeed impressed Washington," said Yang. "Therefore, the US needs to consolidate its alliance with Japan, including their partnership in TPP."

Coming on the occasion of 70th anniversary of the end of World War II, Abe's visit has prompted demands that he use the trip to officially apologize for Japan's historical wrongdoings and especially to the estimated 200,000 women forced into sexual slavery by Japanese troops.

When questioned at the press conference, Abe once more attributed the "comfort women" issue to "human trafficking".

"I am deeply pained to think about the comfort women who experienced immeasurable pain and suffering as a result of victimization due to human trafficking," Abe said in Japanese, repeating the phrase he used during a speech at Harvard University on Monday.

Abe is scheduled to address the US Congress on Wednesday, which will be watched closely by the world, especially by Asian Americans and in Asian countries.