The Asia-Pacific region is now a "top priority" of US security policy, President Barack Obama has said in a speech to the Australian parliament.
Mr Obama insisted US spending cuts would not affect the Asia-Pacific, saying the US is "here to stay".His comments are seen as a challenge to China, which is striving to be the main power in the region.
Mr Obama announced a plan on Wednesday to station a full US Marine task force in Australia by 2016.
The measure will eventually see 2,500 US personnel based in the north of the country.
Defining region
China is locked in a territorial dispute with allies of the US, including the Philippines and Taiwan, over island groupings in the South China Sea.
Beijing has repeatedly insisted that it will discuss the matter only with those countries directly involved, ruling out US involvement.
But analysts say an increased US presence in the area is bound to embolden its allies, and irritate Beijing.
Speaking in Canberra, Mr Obama said Asia-Pacific countries would play a vital role in shaping the world in the 21st Century.
"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 co-operation, needless suffering or human progress," he said.
The US has been slow to recover from an economic slump, and the military is one area earmarked for major spending cuts.
With US troops leaving both Iraq and Afghanistan after long engagements, there was speculation that the Americans might also seek to play a low-key role across Asia.
But Mr Obama said: "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.
"As a result, reductions in US defence spending will not - I repeat, will not - come at the expense of the Asia-Pacific."
He committed the US to playing a "larger and long-term role in shaping this region", before adding that he was keen to work with China.
"We've seen that China can be a partner, from reducing tensions on the Korean peninsula to preventing proliferation," he said.
"We'll seek more opportunities for co-operation with Beijing, including greater communication between our militaries, to promote understanding and avoid miscalculation."
Crocodile insurance
After his speech, the president flew from Canberra to the northern city of Darwin, where many of the US military personnel are likely to be based.In a light-hearted end to his trip, Mr Obama, the first sitting US president to visit the city, was given crocodile insurance as a gift from a local politician.
"I have to admit when we reformed healthcare in America, crocodile insurance is one thing we left out," he joked in an address to about 2,000 soldiers.
The president has now left Australia on his way to a regional summit on the Indonesian island of Bali.
Australian Prime Minister Julia Gillard, who is also under pressure with domestic political issues, said the partnership between Australia and the US had been a "bedrock of stability" in the region.
Mr Obama's trip came as the two countries marked the 60th anniversary of their military alliance.
