WASHINGTON Treasury Secretary Timothy Geithner sharpened his criticism of China's exchange rates on Thursday at a congressional hearing on Beijing's currency policy, one of a lengthening list of issues sparking tension between the two countries.
Below are some questions and answers about the U.S.-China relationship, which is growing more fractious as the two huge powers jostle for political and economic influence around the world.
HOW IMPORTANT IS THE RELATIONSHIP?
President Barack Obama has said the U.S-China relationship will shape the 21st Century, and on nearly every front that is already happening.
Trade between the two countries is flourishing, cross-border investment is increasingly a two-way street and Washington and Beijing are taking halting steps toward diplomatic cooperation on issues such as Iran's nuclear program and the stand-off with North Korea.
The two have squared off over the future of the Internet, the military balance in East Asia, human rights and climate change.
The breadth of the relationship has led some commentators to predict that China and the United States will ultimately become a "Group of 2," setting the global agenda and sidelining the Group of 20 which includes a broad range of developed and developing countries.
Neither the United States nor China has embraced this idea, a notion which alarmed some of their traditional allies.
But it is clear that their uneasy partnership will continue to deepen and grow more complicated as the leaders of the world's largest economy and the world's fastest growing economy seek to figure out the road ahead.
WHAT IS THE CURRENCY DEBATE ABOUT?
Many U.S. lawmakers have charged that China has engineered its economic rise in part by keeping its currency, the yuan, artificially low against the U.S. dollar -- an accusation that Beijing rejects.
But the U.S. trade deficit with China is projected to approach $250 billion this year, and U.S. manufacturers say the yuan needs to appreciate by as much as 25 percent to 40 percent to level the playing field.
Political debate over the currency issue has complicated Obama's efforts to smooth relations with Beijing. While the White House has urged China to take steps to allow the yuan to move more freely, it has stopped short of officially labeling China a currency manipulator, a designation which could lead to possible trade sanctions.
With U.S. voters already frustrated by the struggling economy and stubbornly high unemployment rate near 10 percent, the China currency issue has heated up as Obama's fellow Democrats brace for possibly large losses in the November 2 congressional elections.
The hearings this week at the House Ways and Means, and Senate Banking committees, could result in calls for tough new legislation to punish China. But many analysts say this is a risky approach, which could backfire if China retaliates against U.S. exporters seeking to expand in the world's most populous country.
The U.S. dollar itself is also a hostage to the debate. China's huge $2.45 trillion pile of foreign exchange reserves are almost two-thirds in U.S. dollars, giving Beijing a powerful lever over the dollar's value should it decide to make significant changes.
SIGN OF BROADER PROBLEMS?
Officials in both Washington and Beijing have tried hard to isolate the currency issue, and vowed it would not harm cooperation on other fronts.
There have been some signs that this is working. The United States lobbied successfully for China to back new U.N. sanctions against Iran over its nuclear program, overcoming Beijing's usual reluctance to support punitive measures against one of its key oil suppliers.
China has also cooperated to some degree on North Korea, although progress here has been slower.
Beijing -- the only major ally of Pyongyang's isolated communist government -- has urged North Korea to abandon its nuclear weapons and supported U.N. sanctions over North Korea's atomic violations.
Beijing has also hosted six-party talks with North and South Korea, the United States, Japan and Russia in a bid to resolve the impasse. Those talks stalled in 2009, however, and the issue was further complicated in March when a South Korean naval ship was sunk in what both Seoul and Washington say was a North Korean attack.
Despite heavy U.S. pressure, China stopped short of blaming North Korea for the incident and it is unclear whether the talks can resume soon, as Beijing hopes.
China and the United States are also divided over proposals to fight global climate change -- another Obama priority -- with Beijing saying the developed world should take the lead in cutting carbon emissions.
WHAT ABOUT THE MILITARY DIMENSION?
As its economy boomed, China has also significantly increased its military expenditure -- raising fears of new frictions with the United States particularly over Taiwan and the South China Sea.
Already boasting the largest army in the world, China has also invested in modernizing its navy and combat aircraft to project its power, especially in the Pacific where U.S. forces have long held sway.
Chinese officials point out that their defense spending is still far lower than that of the United States, and that it is not seeking confrontation.
But there have already been rifts. China froze military contacts with the United States after the Obama administration in January unveiled a potential $6.4 billion arms package for Taiwan, which Beijing regards as a renegade province.
While U.S. officials hope this period will soon be over, Taiwan remains a potential flashpoint.
China has also protested joint U.S.-South Korean naval drills after the sinking of the South Korean navy ship and has accused Washington of meddling in the South China Sea, where Beijing is involved in territorial disputes with Southeast Asian nations over an area rich in energy and key to shipping.
WHAT HAPPENED TO HUMAN RIGHTS?
Disputes over human rights once dominated the U.S.-China relationship, but appear to be receding as a public issue.
U.S. officials say they still press China to respect the basic political and religious freedoms of all of its citizens, improve its legal system and end repression of unrest in border areas Tibet and Xinjiang.
While Obama in February met Tibet's exiled spiritual leader, the Dalai Lama, earning sharp Chinese criticism, rights groups say his administration has been less vocal than its predecessors in pushing for political change.
But the human rights issue flared on a new front this year: the Internet. China's Internet controls thrust it into a dispute with search engine giant Google, and Secretary of State Hillary Clinton led U.S. criticism of Beijing's censorship policies, which she said were part of a "new information curtain descending across much of the world."
Analysts say the Internet row -- pitting a U.S. vision of unfettered access against China's more controlled approach -- is a sign of the deep political and cultural differences which continue to divide Beijing and Washington, and which repeatedly lead the two giants to misunderstand each other.
(Reporting by Andrew Quinn; Editing by Tim Dobbyn)