In a speech on the sidelines of the G20 in Brisbane, Mr Obama had strong words for potential aggressor states such as China, Russia, and North Korea, as he told them never to "question our resolve or our commitments to our allies", and lectured them in their own states that the "rule of force must give way to the rule of law".

The strong comments came in a landmark speech to students at Queensland University, ahead of a summit in which the world's biggest economies have agreed to an even more ambitious growth target of 2.1 per cent over five years above business-as-usual levels.

Mr Obama's speech dealt principally with American foreign policy and democracy, but also ventured heavily into climate change, despite the insistence of the Abbott government the issue be left off the G20 agenda.

Fairfax Media understands that climate change will now be specifically mentioned in the final communique with its own reference to the responsibility of member nations to take material action.

That represents a substantial win for Mr Obama, who has succeeded in making the G20 summit address the issue irrespective of the host's reluctance.

Mr Obama said the region was the world's most dynamic and prosperous but also contained pressure points, making reference to territorial tensions between China and Japan.

"Yet alongside this dynamism, we see dangers that could undermine this progress. North Korea's nuclear and missile programs. Disputes over territory – remote islands and rocky shoals – threaten to spiral into confrontation," he said.

The President said all nations had a responsibility to act on climate change, telling Australians in particular they faced more extreme weather including longer droughts and more frequent and severe wildfires as the planet warmed.

The President told the gathering of students, dignitaries, and media that he was yet to visit the Great Barrier Reef.

"I want to come back and I want my daughters to be able to come back and I want them to be able to bring their daughters or sons to visit and I want that there 50 years from now."

He told students, in comments directly linking climate change to bushfires: "Here in Australia it means longer droughts, more wildfires. No nation is immune and every nation has a responsibility to do its part ... which means we've got to step up."

The call for international action follows Mr Obama's G20-eve joint emissions reduction pact with China announced on Wednesday, and his commitment to a $US3 billion ($3.5 billion) contribution to the international Green Climate Fund to help developing nations deal with climate change.

"Now in a historic step China made its own commitment for the first time, agreeing to slow, peak and then reverse the course of China's emissions. And the reason that's so important is because if China as it develops, adapts the same per capita carbon emissions as advanced economies like the US or Australia, this planet doesn't stand a chance because they've got a lot more people."

A draft Brisbane Action Plan to be adopted by all members of G20 is reported to include a plan to lift female workforce participation in what would be an international fillip to Tony Abbott's controversial paid parental leave scheme.

Mr Obama's speech also included a spirited defence of democracy.

"We believe in democracy – that the only real source of legitimacy is the consent of the people; that every individual is born equal with fundamental rights, and that it is the responsibility of governments to uphold these rights. This is what we stand for. This is our vision – the future America is working toward in the Asia-Pacific, with allies and friends."

In a deft repackaging of a familiar argument put forward by previous US presidents, Mr Obama challenged the assumption that democracy was somehow merely a "western" concept, instead arguing it had become a standard in Asia too.

This included nations such as Taiwan, Japan, South Korea, India, and Indonesia.

"As a Pacific power, the United States has invested our blood and treasure to advance this vision. Generations of Americans have served and died here so that the people of the Asia-Pacific might live free," he said.

After announcing the "pivot" to the Asia-Pacific region in the Australian Parliament three years ago, there was considerable interest in Canberra on how that was now being perceived in the White House. Mr Obama said it remained the priority.

"Day in, day out, steadily, deliberately, we will continue to deepen our engagement using every element of our power – diplomacy, military, economic, development and the power of our values," he said.

"I decided that given the importance of this region to American security, to American prosperity, the United States would rebalance our foreign policy and play a larger and lasting role in this region. That's exactly what we've done. Today, our alliances, including with Australia, are stronger than they have ever been. American exports to this region have reached record levels. We've deepened our cooperation with emerging powers and regional organisations, especially in southeast Asia. We expanded our partnerships with citizens as they work to bolster their democracies."