Prime Minister Manmohan Singh, right, says India offers a trade alternative to China, and its prime minister, Wen Jiabao.

By MARK LANDLER, JIM YARDLEY and MICHAEL WINES

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 scheduled trip this weekend to Asia , his most extensive as president, was set to take him to the area’s big democracies, first to India, then Indonesia, South Korea and Japan, skirting 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 were expected to sign a landmark deal for American military transport aircraft and discuss the possible sale of jet fighters. Japan and India are courting Southeast Asian nations with trade agreements and talk of a “circle of democracy.” And 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 mostly southeast Asian countries gathered in late October in Hanoi for a meeting suffused by tensions between China and its neighbors.

China’s feud with Japan over islands in the East China Sea stole the meeting’s headlines, and Secretary of State Hillary Rodham Clinton proposed a three-way negotiations to resolve the issue. Also, 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 China’s 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 on October 28 in Hawaii, before opening a seven-country tour of Asia that included a last-minute stop in China.

“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 of public policy.

As Mr. Obama prepare d to start his trip in India , Mr. Singh would have just returned from his own grand tour of the region ? with both of them somewhat conspicuously circling China.

But 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.

Still, India is promoting itself 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 again wield influence.

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. 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.” 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.

In late October, People’s Daily, the Communist Party newspaper, ran an opinion article asking, “Does India’s ‘Look East’ Policy Mean ‘Look to Encircle China’?” “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.”