HANOI, Vietnam — China’s military expansion and assertive trade policies have set off jitters across Asia, prompting many of its neighbors to rekindle old alliances and cultivate new ones to better defend their interests against the rising superpower.

A whirl of deal-making and diplomacy, from Tokyo to New Delhi, is giving the United States an opportunity to reassert itself in a region where its eclipse by China has been viewed as inevitable.

President Obama’s trip to the region this week, his most extensive as president, will take him to the area’s big democracies, India, Indonesia, South Korea and Japan, skirting authoritarian China. Those countries and other neighbors have taken steps, though with varying degrees of candor, to blunt China’s assertiveness in the region.

Mr. Obama and Prime Minister Manmohan Singh of India are expected to sign a landmark deal for American military transport aircraft and are discussing the possible sale of jet fighters, which would escalate the Pentagon’s defense partnership with India to new heights. Japan and India are courting Southeast Asian nations with trade agreements and talk of a “circle of democracy.” Vietnam has a rapidly warming rapport with its old foe, the United States, in large part because its old friend, China, makes broad territorial claims in the South China Sea.

The deals and alliances are not intended to contain China. But they suggest a palpable shift in the diplomatic landscape, on vivid display as leaders from 18 countries gathered this weekend under the wavelike roof of Hanoi’s futuristic convention center, not far from Ho Chi Minh’s mausoleum, for a meeting suffused by tensions between China and its neighbors.

China’s escalating feud with Japan over another set of islands, in the East China Sea, stole the meeting’s headlines on Saturday, and Secretary of State Hillary Rodham Clinton proposed three-way negotiations to resolve the issue.

Most Asian countries, even as they argue that China will inevitably replace the United States as the top regional power, have grown concerned at how quickly that shift is occurring, and what China the superpower may look like.

China’s big trading partners are complaining more loudly that it intervenes too aggressively to keep its currency undervalued. Its recent restrictions on exports of crucial rare earths minerals, first to Japan and then to the United States and Europe, raised the prospect that it may use its dominant positions in some industries as a diplomatic and political weapon.

And its rapid naval expansion, combined with a more strident defense of its claims to disputed territories far off its shores, has persuaded Japan, South Korea, Vietnam and Singapore to reaffirm their enthusiasm for the American security umbrella.

“The most common thing that Asian leaders have said to me in my travels over this last 20 months is, ‘Thank you, we’re so glad that you’re playing an active role in Asia again,’ ” Mrs. Clinton said in Hawaii, opening a seven-country tour of Asia that included a last-minute stop in China.

Few of China’s neighbors voice their concerns about the country publicly, but analysts and diplomats say they express wariness about the pace of China’s military expansion and the severity of its trade policies in private.

“Most of these countries have come to us and said, ‘We’re really worried about China,’ ” said Kenneth G. Lieberthal, a China adviser to President Bill Clinton who is now at the Brookings Institution.

The Obama administration has been quick to capitalize on China’s missteps. Where officials used to speak of China as the Asian economic giant, they now speak of India and China as twin giants. And they make clear which one they believe has a closer affinity to the United States.

“India and the United States have never mattered more to each other,” Mrs. Clinton said. “As the world’s two largest democracies, we are united by common interests and common values.”

As Mr. Obama prepares to visit India in his first stop on his tour of Asian democracies, Mr. Singh, India’s prime minister, will have just returned from his own grand tour — with both of them somewhat conspicuously, if at least partly coincidentally, circling China.

None of this seems likely to lead to a cold war-style standoff. China is fully integrated into the global economy, and all of its neighbors are eager to deepen their ties with it. China has fought no wars since a border skirmish with Vietnam three decades ago, and it often emphasizes that it has no intention of projecting power through the use of force.

At the same time, fears that China has become more assertive as it has grown richer are having real consequences.

India is promoting itself throughout the region as a counterweight to China; Japan is settling a dispute with the United States over a Marine air base; the Vietnamese are negotiating a deal to obtain civilian nuclear technology from the United States; and the Americans, who had largely ignored the rest of Asia as they waged wars in Afghanistan and Iraq, see an opportunity to come back in a big way.

In July, for example, Mrs. Clinton reassured Vietnam and the Philippines by announcing that the United States would be willing to help resolve disputes between China and its neighbors over a string of strategically important islands in the South China Sea.

China’s foreign minister, Yang Jiechi, reacted furiously, accusing the United States of plotting against it, according to people briefed on the meeting. Mr. Yang went on to note that China was a big country, staring pointedly at the foreign minister of tiny Singapore. Undaunted, Mrs. Clinton not only repeated the American pledge on the South China Sea in Hanoi on Saturday, but expanded it to include the dispute with Japan.

China’s rise as an authoritarian power has also revived a sense that democracies should stick together. K. Subrahmanyam, an influential strategic analyst in India, noted that half the world’s people now live in democracies and that of the world’s six biggest powers, only China has not accepted democracy.

“Today the problem is a rising China that is not democratic and is challenging for the No. 1 position in the world,” he said.

Indeed, how to deal with China seems to be an abiding preoccupation of Asia’s leaders. In Japan, Prime Minister Naoto Kan and Mr. Singh discussed China’s booming economy, military expansion and increased territorial assertiveness.

“Prime Minister Kan was keen to understand how India engages China,” India’s foreign secretary, Nirupama Rao, told reporters. “Our prime minister said it requires developing trust, close engagement and a lot of patience.”

South Korea was deeply frustrated earlier this year when China blocked an explicit international condemnation of North Korea for sinking a South Korean warship, the Cheonan. South Korea accused North Korea of the attack, but China, a historic ally of the North, was unwilling to hold it responsible.

India has watched nervously as China has started building ports in Sri Lanka and Pakistan, extending rail lines toward the border of Nepal, and otherwise seeking to expand its footprint in South Asia.

India’s Defense Ministry has sought military contacts with a host of Asian nations while steadily expanding contacts and weapons procurements from the United States. The United States, American officials said, has conducted more exercises in recent years with India than with any other nation.

Mr. Singh’s trip was part of his “Look East” policy, intended to broaden trade with the rest of Asia. He has said it was not related to any frictions with China, but China is concerned. On Thursday, People’s Daily, the Communist Party newspaper, ran an opinion article asking, “Does India’s ‘Look East’ Policy Mean ‘Look to Encircle China’?”

That wary view may well reflect China’s reaction to the whole panoply of developments among its neighbors.

“The Chinese perceived the Hanoi meeting as a gang attack on them,” said Charles Freeman, an expert on Chinese politics and economics at the Center for Strategic and International Studies. “There’s no question that they have miscalculated their own standing in the region.”

Mark Landler reported from Hanoi, Jim Yardley from New Delhi, and Michael Wines from Beijing.