The United States joins an East Asian pact this week, yoking a diplomatic offensive to assert itself as a Pacific power with the interests of countries wary of China's emerging might.

The 16-nation East Asia Summit, which already includes China, India and Japan, will expand by two members as US President Barack Obama formally takes his seat together with Russia, expected to be represented by Foreign Minister Sergei Lavrov.

At the same time global hopes for an economic recovery in the US and other developed countries -- including the flailing eurozone -- are pinned on Asia, where robust economies have both the power to buy goods and invest overseas.

Indonesia's Bali island, where the meeting is taking place, is normally a haven of tourist relaxation but has been transformed for the event, with six warships patrolling off the beaches and 7,000 police and soldiers on duty.

The EAS forum is held annually in conjunction with a summit of the 10-nation Association of Southeast Asian Nations (ASEAN), which welcomes a more robust US role in the Pacific to counter China's growing military and economic influence.

Obama will arrive from Australia, where he is expected to announce the deployment of US marines at a base in the northern city of Darwin -- a signal that Washington will defend its regional interests and allies.

And several ASEAN members locked in territorial disputes with Beijing over the resource-rich South China Sea have welcomed intervention from Washington, which has backed their campaign for a multilateral solution based on international law.

"It is not by design that someone is invited in to counter anyone else. But in the balance, that is how it's going to work itself out," ASEAN Secretary-General Surin Pitsuwan told AFP.

"We certainly welcome the addition. Rather than keeping the elephant outside the tent it is better to bring it in, so that it has ownership of it and helps strengthen the tent."

Both the United States and ASEAN have an interest in enlarging the East Asia Summit, said Tim Huxley, executive director for the International Institute of Strategic Studies in Singapore.

"From the Asian perspective there is a widespread appreciation of China's economic rise and how countries can benefit from that, but also substantial concern... about China's political and military rise," he said.

Secretary of State Hillary Clinton, who will also be in Bali, outlined at the weekend's Asia-Pacific Economic Cooperation (APEC) forum in Honolulu where US foreign policy interests lie as it disengages from Iraq and Afghanistan.

"One of the most important tasks of American statecraft over the next decade will be to lock in a substantially increased investment -- diplomatic, economic, strategic, and otherwise -- in the Asia-Pacific region," she said.

But there are concerns that a greater US role in what China sees as its sphere of influence could be a source of future discord.

The Philippines said this week it was looking to form a united front among ASEAN nations on the South China Sea wrangle, an initiative opposed by Beijing which prefers to negotiate individually with its weaker neighbours.

At APEC Obama betrayed increasing frustration over China's trade and currency practices, saying Beijing has not done enough to allow the yuan to reach a fair market level.

"We don't try to game the system. Thats part of what leadership is about," he said, adding that while breaking rules may have been acceptable decades ago, China had now "grown up" economically and should behave in a responsible way.

Obama also laid out his vision for the Trans-Pacific Partnership (TPP), an ambitious trade grouping which could encompass 12 nations, but not yet China, the world's second-largest economy.

Beijing has previously protested the US-driven TPP as overly ambitious, but foreign ministry official Pang Sen said in Honolulu that China would "earnestly study" whether to seek membership of the proposed free-trade zone.

Richard Cronin of the Stimson Center in Washington said that despite existing tensions between the world's two largest economies, the two have interests that transcend competition.

"I don't see US reengagement in the Asia-Pacific as a new chapter in a rivalry with China," he told AFP. "The US and China are still economically interdependent... We have important enduring interests that need attention."