President Barack Obama signalled a pivotal US shift to Asia on Thursday, pledging not to let Washington's budget crunch compromise his expansive vision and military presence in the region.

US President Barack Obama addresses the Australian Parliament in Canberra on November 17, 2011.  (AFP Photo/Rick Rycroft)

"The United States is a Pacific power, and we are here to stay," Obama declared in the Australian parliament on the latest stop of a tour that he said augured a "larger and long-term role" in shaping Asia.

His commitment came a day after the United States said it would deploy up to 2,500 Marines to northern Australia and tighten air force cooperation, sparking concern from China, whose rapid rise is reorienting Asia's strategic balance.

In a message aimed both at a dynamic region he sees as key to the US economic future and budget-slashing Republicans at home, Obama said the Asia-Pacific was too vital to fall prey to US penny-pinching.

"As the United States puts our fiscal house in order, we are reducing our spending," Obama said, warning that reductions in funding for the US military machine were inevitable after years of huge spending in Iraq and Afghanistan.

But he added: "Here is what this region must know. 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.

"Reductions in US defence spending will not -- I repeat, will not -- come at the expense of the Asia-Pacific."

But Obama's promise came at an inopportune moment, as by coincidence the US national debt passed $15 trillion for the first time hours before he spoke, prompting Republicans to slam the president for profligate spending.

Analysts have said key players in the region are weighing Obama's promise of a long-term commitment against his debt-laden nation's capacity to fund carrier battle groups, garrisons in Japan and Korea, and new troop deployments.

"The United States will continue our effort to build a cooperative relationship with China," Obama said in the signature speech of a tour that has laid bare divisions between Washington and Beijing.

"All of our nations have a profound interest in the rise of a peaceful and prosperous China -- and that is why the United States welcomes it," he added.

After his speech, Obama joined Australian Prime Minister Julia Gillard at a Canberra school, greeted children with a hearty "G'day" and promised he would say "hi" to teen singing sensation Justin Bieber.

He told the kids he delighted meeting children as they were not stuck in "old stodgy ideas".

Obama's comments on funding represent a landmark moment in US policy towards a region seething with territorial disputes, containing trade routes vital to US prosperity and which is transfixed by the rise of China.

But his own defence secretary has warned that a possible $500 billion of new cuts to the Pentagon budget could jeopardise US operations overseas, raising the questions of where Obama will slim the military, if Asia is immune.

Obama later headed to Darwin, where he was to address US and Australian troops, highlight the US Marines deployment that begins next year, and pay homage to the 60-year old US-Australia security alliance.

In the tropical northern outpost, he visited the memorial of the USS Peary, which honours the 89 men killed when the warship was bombed by the Japanese during World War II raids on the port.

In his speech, Obama also made a case to Americans back home as he eyes a tough reelection fight at a time of nine percent unemployment and argues trade with emerging Asian markets could be the key to future US prosperity.

"As the world's fastest-growing region -- and home to more than half the global economy -- Asia is critical to achieving my highest priority: creating jobs and opportunity for the American people," he said.