US President Barack Obama and Australian Prime Minister Julia Gillard today unveiled plans to deepen the US military presence in the Asia-Pacific, with 2,500 US marines operating out of a de facto base in northern Australia.

China, already worried the United States is caging it in, immediately questioned whether strengthening military alliances would help the region when economic woes put a premium on cooperation.

"With my visit to the region, I am making it clear that the United States is stepping up its commitment to the entire Asia-Pacific region," Obama told a joint news conference with Gillard in Canberra.

From next year, US troops and aircraft will operate out of the tropical city of Darwin, only 820 km from Indonesia, able to respond quickly to any humanitarian and security issues in Southeast Asia, where disputes over sovereignty of the South China Sea are causing rising tensions.

"It is appropriate for us to make sure...that the security architecture for the region is updated for the 21st century and this initiative is going to allow us to do that," Obama said.

He stressed that it was not an attempt to isolate China which is concerned that Washington is trying to encircle it with bases in Japan and South Korea and now troops in Australia.

"The notion that we fear China is mistaken. The notion that we are looking to exclude China is mistaken," he said, adding China was not being excluded from the planned Trans-Pacific Partnership (TTP) on trade. "We welcome a rising, peaceful China."

But China's rising power means it must take on greater responsibilities to ensure free trade and security in the region, he added. "It's important for them to play by the rules of the road and, in fact, help underwrite the rules that have allowed so much remarkable economic progress," he said