By Laura MacInnis and Caren Bohan
HONOLULU (Reuters) - President Barack Obama will unveil plans to deepen America's military presence in the Asia-Pacific region during a trip starting on Wednesday to Australia, where he aims to bolster ties with a staunch U.S. ally.
The winding down of wars in Afghanistan and Iraq has opened the door to greater U.S. attention to simmering tension over the South China Sea, a shipping lane for more than $5 trillion in annual trade that the United States wants to keep open.
Fresh from hosting an Asia-Pacific trade summit in his native Hawaii, Obama is set to outline his vision for an expanded U.S. role in the region in an address to Australia's parliament in Canberra on Thursday.
He will then travel to the remote tropical city of Darwin -- a gateway to Southeast Asia -- and announce plans for thousands of U.S. maritime forces to train and do more joint exercises with the Australian military on the country's northern coast.
The agreement will not include any permanent U.S. basing, but an Obama administration official said it was a stepping stone to a more stable presence in Australia, which offers closer access to the South China Sea than America's bases in Japan and South Korea.
Robert Willard, head of the U.S. Pacific command, said it was currently necessary to deploy forces across great distances to reach the shipping lane, which he described as "incredibly vital to the region, to our partners and allies, and certainly to the United States."
"Any opportunities that we have to locate forces in the Southeast Asia region relieves some pressure on that need to, at great expense, deploy and sustain forces present in Southeast Asia," Willard told reporters in Honolulu.
"Any rebalancing that can take place over time to permit the United States to more effectively be present in the region I think is a positive step," the commander said.
China claims the whole of the South China Sea although Vietnam, the Philippines, Taiwan, Malaysia and Brunei hold rivals claims to at least parts of it. Tension occasionally flares up into maritime stand-offs.
Australia says hosting U.S. troops on Australian soil and the pre-positioning of U.S. supplies in Darwin is not the precursor to a U.S. base, but Australia's largest export market China may see it as further moves to encircle it.
"I think it well and truly possible for us in this growing region of the world to have an ally in the United States and to have deep friendships in our region, including with China," Australian Prime Minister Julia Gillard said on Tuesday.
China's hunger for raw materials for its booming economy has cocooned Australia from the global economic downturn. China is Australia's biggest export market worth A$58 billion ($59 billion) in 2010, mainly from iron ore and coal sales.
Australia and the United States jointly operate an intelligence base at
Pine Gap in the outback and routinely take part in military exercises. But the Darwin deployment will be the largest in Australia since World War Two when General Douglas MacArthur moved his war headquarters there.
Deeper U.S.-Australia military ties should come as no surprise to China, given their alliance has seen their troops fight side by side in every major war , say analysts .
Darwin, nicknamed the "Pearl Harbour of Australia" after a World War Two Japanese raid dropped more bombs on the city than Japan dropped on Pearl Harbour, is only 820 km (500 miles) from Indonesia and with open access to the Indian Ocean.
"I think it will be something along the lines of the Singapore model ,"
said Rory Medcalf at the Lowy Institute for International Policy in Sydney.
" The U.S. has constant ship visits in and out of Singapore, it doesn't actually have a military base in Singapore, but it has such a constant presence in Singapore that it is almost the same thing," said Medcalf.
"There will be a regular U.S. presence that will obviously be on call in a future security crisis.
I think this is meant to complement the U.S. presence in Japan, Korea and elsewhere ."
In Canberra, Obama is likely to describe a range of U.S. strategic interests in Asia, including regional concerns about human and drug trafficking, piracy, violent extremism as well as the future of the Korean peninsula.
The United States is also looking to improve its military relationships with India, another key emerging economy with strong maritime and commercial interests in the region, and speed its response to humanitarian disasters across Asia.
"T he prospect of increased U.S. access to military bases in Australia ... would provide a building block for trilateral cooperation with India in the Indian Ocean ," said the Washington-based Heritage Foundation in a recent report on US-Australia-India strategic relations.
" India is slowly shifting its focus eastward to cope with expected increasing Chinese naval activity in the Bay of Bengal and Indian Ocean and to enable India to project power into the Asia-Pacific. "
After the Australia tour, which marks his first visit as president, Obama will travel to Bali as the first U.S. president to attend the East Asian Summit, a diplomatic bloc that is admitting the United States and Russia this year.
There, he will seek to build on growing U.S. trade ties with smaller powers such as Thailand, Malaysia, Vietnam, the Philippines and Indonesia and make clear that the United States is looking out for them with its greater focus on Asia.
Charles Kupchan, a Georgetown University professor, said Southeast Asian nations as well as Japan and South Korea were likely to welcome greater U.S. engagement in the region as a counterweight to China.
"They realize that China will be the dominant economic power in the region, and it is unclear what role China will play geopolitically," Kupchan said. "They are, therefore, looking to refurbish their relationships with the United States as a way of balancing against Chinese power."
(Additional reporting by Michael Perry in Sydney; Editing by Robert Birsel)