CANBERRA - US President Barack Obama announced on Thursday that the US military would expand its role in the Asia-Pacific, despite budget cuts, declaring America was "here to stay" as a Pacific power which would help shape the region's future.

The US military, turning its focus away from Iraq and Afghanistan, would be more broadly distributed in Asia, particularly Southeast Asia, more flexible and help build regional capacity, Obama told the Australian parliament.

"As we end today's wars, I have directed my national security team to make our presence and missions in the Asia Pacific a top priority," Obama said in a major speech on Washington's vision for the Asia-Pacific.

"As a result, reductions in US defense spending will not -I repeat, will not - come at the expense of the Asia Pacific."

Acknowledging China's concerns at the US moves, which Beijing sees as attempts by Washington to encircle it, Obama said he would seek greater cooperation with China.

"We'll seek more opportunities for cooperation with Beijing, including greater communication between our militaries to promote understanding and avoid miscalculation," he said.

"We will preserve our unique ability to project power and deter threats to peace," said Obama.

"With most of the world's nuclear powers and some half of humanity, Asia will largely define whether the century ahead will be marked by conflict or cooperation," he said.

"As a Pacific nation, the United States will play a larger and long-term role in shaping this region and its future..."

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A first step in extending the US military reach into Southeast Asia will see US marines, naval ships and aircraft deployed to northern Australia.  

Obama and Australian Prime Minister Julia Gillard on Wednesday announced plans to have 2,500 US Marines operate out of a de facto base in the northern port of Darwin by 2016.  

Obama was to discuss those plans later on Thursday in a visit to Darwin, only 820 kms (500 miles) from Indonesia. He and Gillard will salute the 60-year US-Australian alliance by laying a wreath at a memorial for the USS Peary, a US destroyer that was sunk by the Japanese during World Ward Two.

White House aide Ben Rhodes said the United States was sending a signal "that we're going to continue to play the role of underpinning security in this part of the region."

But Obama and his aides have stressed the United States was not seeking to isolate China or ratchet up tensions.

Obama's Australia visit falls midway through Obama's nine-day Asia-Pacific tour that takes him next to Bali, where he will seek to underscore a focus on Asia by becoming the first US president to participate in the security East Asia Summit.

He flew to Australia from Hawaii, where he hosted an Asia-Pacific Economic Cooperation summit that focused on bolstering trade and economic ties within the region.