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 a group of major powers that includes the United States, Europe and China. 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 U.N. 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 U.N. statement condemning the North’s recent attempt to launch a satellite.

And there is progress on the economic front: U.S. officials said China has 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 U.S. 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 Barack 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 U.S. 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 U.S. warships, which both prompted public rebukes from China.

The White House has also thrown a blanket of silence over the role a U.S. 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, 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.”

Clinton’s choice of words was noteworthy. A new appraisal of the U.S.-Chinese relationship by Brookings concludes there is deep-seated distrust between the two countries. Beijing, in particular, views the relationship as a “long-term zero-sum game,” the report said.

U.S. officials are realistic about the limits to cooperation between China and United States. Though China joined in the United Nations’ rebuke of North Korea after its missile launching, President Hu Jintao later welcomed a top official of the ruling Workers’ Party to Beijing, while another senior Chinese official hailed the relationship between Beijing and Pyongyang as a “precious gem.”

And North Korea may carry out its vow to conduct a nuclear test. In the aftermath of the failed satellite launching, North Korea staged a military parade featuring a missile-launching vehicle that administration officials say they think was sold to the North by a Chinese manufacturer. While the United States does not believe the Chinese government willfully violated a U.N. ban on military sales to Pyongyang, officials say the sale demonstrates China’s inconsistent approach to enforcing it. “I don’t think China is going to do anything to stop, or get to know, which companies are involved in this,” said Victor Cha, who negotiated with North Korea during the George W. Bush administration. “We should identify a few of these companies, and sanction them.”

China’s calculations on Iran are just as complex. It is Iran’s biggest energy customer, accounting for more than a fifth of its oil exports. But under U.S. sanctions law, it will be subject to punitive measures at the end of June, unless it shows a “significant reduction” in its imports from Iran or wins a waiver from Obama on national security grounds.

The Chinese government wants to avoid those punitive measures, U.S. officials and Western diplomats said. Given Iran’s isolation, analysts said China may also have concluded that it should diversify its sources of supply. And U.S. officials said China shares the urgency of the United States and Europe in preventing Iran from obtaining a nuclear bomb.

The Chinese, said Clifford Kupchan, the Middle East director for the Eurasia Group, a Washington consultancy, “have been bad actors” in the years-long diplomacy over Iran, though in recent weeks “their diplomatic rhetoric is tougher and their oil purchases are lower,” he said.

The question is whether China is simply waiting out Iran to extract a better price. With Japan and South Korea also cutting purchases to avoid U.S. sanctions, Iran is being forced to stockpile oil in tankers anchored in the Persian Gulf. Unless it shuts down its oil wells, analysts say Iran will run out of storage capacity by summer.

That is when China’s intentions will become clearer. By then, however, Obama will have had to make a critical decision on whether to exempt China from the new sanctions.

© 2012 The New York Times Company