That complicates the strategic picture for the Obama administration as it looks to advance its work in Asia and strengthen not just its own alliances, but get its partners in the region to collaborate more.

Vice-President Joe Biden will broach these issues when he travels to Japan, China and South Korea next week - a trip to demonstrate that the top level of the administration remains focused on Asia.

Obama's shift in focus - once known as a pivot, now called a rebalance - has always seemed more rhetorical than real.

"What isn't clear to me is whether they see this as a Japan-China problem that needs to be managed or as part of a longer-term test of wills with Beijing," said Michael Green, an Asia adviser in the George W. Bush administration who is now at the Centre for Strategic and International Studies.

Green said the United States needs to project military power in the region, build up the defensive capacities of allies like Japan and the Philippines, and align the countries that ring China's coastal waters to present a united front against Beijing's aggression.

The trouble, he added, is that "the administration is very worried about appearing to contain China".

Others echoed the concern. "The region is moving in a very problematic direction," said Evans Revere, a former senior US diplomat and East Asia specialist. "That's the result of territorial disputes, historical issues, long-standing rivalries and the inability of countries to put history behind them and move forward in improving relations."

Adding to this witches' brew of bickering in the region, Washington is grappling with the threat posed by an unpredictable North Korea. The deal the US orchestrated with Iran to temporarily freeze its nuclear programme, despite three decades of animosity, is a stark reminder of the impasse in negotiations with Pyongyang.

Unlike Iran, North Korea already has a nuclear bomb, and there's worrying evidence it is developing weapons.

The administration said Biden would meet Chinese leaders including President Xi Jinping and would voice concern about what it called an emerging pattern of behaviour by China that is unsettling to its neighbours. The vice-president will also make clear its desire to lower tensions between China and Japan, the world's second- and third-largest economies.

Secretary of State John Kerry hasn't neglected the region, but his primary focus remains the Middle East and is likely to stay there as he strives for the distant goals of an end to Syria's civil war, peace between Israelis and Palestinians, and a nuclear agreement with Iran after the current pact expires in six months.

US domestic woes have contributed to a sense that Asia is a secondary concern to the administration.

Obama was forced to cancel a four-nation trip to the region in October because of a partial US government shutdown and threat of a debt default. He'll travel to Asia in April instead.

Obama made Asia a foreign policy priority when he took power in 2009 and has been active in engaging China. Not known for a personal touch with foreign leaders, Obama sought to cultivate a relationship with new leader Xi when he met him in June at a California resort. That's part of a strategy to promote co-operation between the world's two largest economies and prevent their rivalry in the Asia-Pacific from spawning conflict.

But China's declaration of its East China Sea air defence zone will be viewed as unhelpful. It was rejected by Japan, South Korea and Taiwan, and prompted quick expressions of deep US concern that it could escalate tensions in the region. "This really casts a bit of a pall over efforts to improve [bilateral] relations," Revere said.

The US said it would not change how it conducts military operations in the region and flew a pair of B-52 bombers through the zone on Tuesday on what officials said was a long-planned training mission. China's Defence Ministry said it had detected and monitored the bombers. It said all aircraft flying through the zone would be monitored, but made no mention of a threat to take "defensive emergency measures" against non-compliant aircraft that was included in an announcement on Saturday.

Bonnie Glaser, a China specialist at the Centre for Strategic and International Studies, expects Biden to raise the issue with Chinese leaders. She said that while countries have a right to declare such a zone - the US, South Korea and Japan all have them - there is concern as to how China will enforce it.

"The question is how many times China will scramble their jets and against whom," she said.

The zone encompasses unoccupied but Japanese-administered islands that Japan calls the Senkakus and China calls the Diaoyus. Since Japan nationalised some of the islands a year ago, there's been a constant cat-and-mouse between the two nations' sea vessels and aircraft.

There's been no skirmish, although Japan accused China in January of locking targeting radar on a Japanese helicopter and frigate, which underscored the risks of a clash.

Proponents of the US pivot view a strong American military presence and diplomatic engagement as essential to maintaining the decades of relative stability and economic prosperity the region has enjoyed.

But the rift between South Korea and Japan, which host some 80,000 US military personnel between them, complicates that task. Japanese leader Shinzo Abe's intent to allow a more active role for its military, which is constrained by a pacifist constitution, has further alienated South Korea.

Victor Cha, White House director for Asia affairs under President George W. Bush, said he had raised concerns that Seoul is siding with Beijing on the issue. Although Seoul has criticised the new Chinese air defence zone, he said the Obama administration faces a major strategic problem: "How do you pivot to Asia when your two main allies are deeply in conflict with each other?"