WASHINGTON — When China suddenly began cutting back its purchases of oil from Iran in the last month,  officials in the Obama administration were guardedly optimistic, seeing the move as  the latest in a string of encouraging signs from Beijing on sensitive security issues  like Syria and North Korea, as well as on politically fraught economic issues like China’s exchange rate.


  As with so many signals from Beijing, though, its underlying motives for reducing its imports of Iranian oil remain a mystery: Are the Chinese embracing Western sanctions? Or, as some experts suspect, are they trying to extract a better price from one of their main suppliers of crude?


 The answer is probably a bit of both, according to senior administration officials who acknowledge that they do not know for certain. But for the White House, which has labored to build a more constructive relationship with China, Beijing’s motives may matter less than the general direction in which it appears to be moving.


For years, China  stymied efforts to pressure Iran. Now, in addition to  throwing its weight behind the sanctions effort, officials say,  Beijing is also playing a more active role in the recently revived nuclear talks between Iran and six world powers — the United States, China, Russia, Britain, France and Germany. While in past negotiations, Beijing has followed in lockstep the positions taken by Russia, this time Chinese diplomats are offering their own proposals.


“One of the key elements of making this work is unity among the major powers,” said a senior administration official, who spoke on condition of anonymity to discuss diplomatic exchanges. “The Chinese have been very good partners in this regard.”


There are also signs of new cooperation on Syria. Only weeks after Secretary of State Hillary Rodham Clinton called China’s veto of a United Nations Security Council resolution “despicable,” China is supporting Kofi Annan’s peace plan for the strife-torn country and is deploying monitors to help oversee it. Even on North Korea, which China has long sheltered from tougher international action, the Chinese government quickly signed on to a United Nations statement condemning the North’s recent attempt to launch a satellite.


And there is progress on the economic front: American officials said China recently loosened trading on its currency, the renminbi, which could help close a valuation gap with the dollar that has stoked trade tensions between China and the United States during an election year.


To some seasoned observers of China, these developments are less a harbinger of a new era of cooperation between Beijing and Washington than evidence that, at least for now, the interests of the two countries coincide in some important areas. And these positive signs come despite new American efforts to bolster its troop presence and military alliances to counter China’s dominance in the region.


“Over time, there are interests that overlap to some degree and differ to some degree,” said Jeffrey A. Bader, a former China adviser to President Obama. “The relationship tends to move up and down over time, as if along a sine curve. But the recent story is mostly a positive one.”


With American and Chinese officials preparing for high-level consultations in Beijing next week, the Obama administration is accentuating these positive developments and playing down potential sources of friction like the recent announcement that it would station 2,500 Marines in Australia and the talks it has begun with the Philippines to conduct more joint military exercises and allow more frequent visits by American warships, which both prompted public rebukes from China.


The White House has also thrown a blanket of silence over the role an American consulate played in briefly harboring a former associate of the deposed Communist Party official Bo Xilai.


Speaking at the Brookings Institution last week, Treasury Secretary Timothy F. Geithner said, “The cumulative effect of what China has done is very significant and very promising.” At the Naval Academy earlier in the month, Mrs. Clinton declared, “Geopolitics today cannot afford to be a zero-sum game; a thriving China is good for America and a thriving America is good for China.”