US President Barack Obama has said that Asia's security must not be based on intimidation where big nations bully small ones.
Mr Obama told students in Brisbane, Australia - where he is attending the G20 summit - that security in the region must depend on mutual alliances.
He said there was "no question" over US commitment to its Asia-Pacific allies.
The two-day G20 summit, which is getting under way, will focus on promoting growth.
World Leaders are expected to expand on plans agreed in February at the G20 finance ministers' meeting to boost global economic growth by 2% in five years.
Speaking just ahead of the meeting, the UN Secretary-General Ban Ki-Moon also urged those attending to meet the challenges posed by Ebola, climate change and the unresolved conflict in Ukraine.
Russian President Vladimir Putin is expected to face a hostile reception from some Western leaders concerned about Russia's increasing military assertiveness.
Mr Obama told students in Brisbane, Australia - where he is attending the G20 summit - that security in the region must depend on mutual alliances.
He said there was "no question" over US commitment to its Asia-Pacific allies.
The two-day G20 summit, which is getting under way, will focus on promoting growth.
World Leaders are expected to expand on plans agreed in February at the G20 finance ministers' meeting to boost global economic growth by 2% in five years.
Speaking just ahead of the meeting, the UN Secretary-General Ban Ki-Moon also urged those attending to meet the challenges posed by Ebola, climate change and the unresolved conflict in Ukraine.
Russian President Vladimir Putin is expected to face a hostile reception from some Western leaders concerned about Russia's increasing military assertiveness.
Speaking on the fringes of the summit on Saturday, President Obama said Russia's "aggression" towards Ukraine was a "threat to the world" - citing the downing of Malaysia Airlines flight MH17 as proof.
Security in the Asia-Pacific region, meanwhile, must depend on international law and the peaceful resolution of disputes, he said.
"We believe... that any effective security order for Asia must be based not on spheres of influence, or coercion or intimidation where big nations bully the small, but on alliances for mutual security, international law and norms that are upheld, and the peaceful resolution of disputes," Mr Obama told students at Brisbane's University of Queensland.
He warned of the dangers posed by territorial disputes in the South China Sea, where what is seen as Beijing's assertiveness has raised concern among its neighbours.
He also used the speech to announce that the US would contribute $3bn (£1.9bn) to an international fund to help poor countries cope with the effect of climate change.
'Any subject'Earlier Australian Prime Minister Tony Abbott said world leaders would use the summit to discuss job creation, identifying tax cheats and strengthening the global economy.
His government had tried to keep climate change off the agenda, despite calls from campaigners.
But speaking to leaders on Saturday, Mr Abbott said: "Obviously I would like this discussion to focus on the politics of economic reform.
"In the end, though, this is your retreat, it is open to any of you to raise any subject that you wish."
Leaders of the G20 nations are expected to focus on economic growth
Security in the Asia-Pacific region, meanwhile, must depend on international law and the peaceful resolution of disputes, he said.
"We believe... that any effective security order for Asia must be based not on spheres of influence, or coercion or intimidation where big nations bully the small, but on alliances for mutual security, international law and norms that are upheld, and the peaceful resolution of disputes," Mr Obama told students at Brisbane's University of Queensland.
He warned of the dangers posed by territorial disputes in the South China Sea, where what is seen as Beijing's assertiveness has raised concern among its neighbours.
He also used the speech to announce that the US would contribute $3bn (£1.9bn) to an international fund to help poor countries cope with the effect of climate change.
'Any subject'Earlier Australian Prime Minister Tony Abbott said world leaders would use the summit to discuss job creation, identifying tax cheats and strengthening the global economy.
His government had tried to keep climate change off the agenda, despite calls from campaigners.
But speaking to leaders on Saturday, Mr Abbott said: "Obviously I would like this discussion to focus on the politics of economic reform.
"In the end, though, this is your retreat, it is open to any of you to raise any subject that you wish."
In early developments:
Brisbane, in the state of Queensland, is said to be taking unprecedented security measures for the summit, with some 6,000 police deployed.
Protesters took to Sydney's Bondi Beach to make a statement about the global climate change debate
Twenty-seven different groups have been given permits to protest at designated areas near the Brisbane Convention and Exhibition Centre, the venue hosting the meeting, and thousands of people are expected over the weekend.
More than 200 protesters buried their heads in the sands of Bondi Beach on Thursday in a demonstration over climate change inaction.
The Australian government is facing criticism over its climate policies. Since coming to power, Mr Abbott has axed a carbon tax and overseena 70% fall in investment in renewable energy.
- British Prime Minister David Cameron has outlined new measures to tackle jihadists who fight abroad, in an address to the Australian parliament
- Japanese Prime Minister Shinzo Abe called for closer tripartite defence ties with the US and Australia, and urged "tangible steps taken in the area of security and defence cooperation" with Canberra, in a piece for the Australian Financial Review
Brisbane, in the state of Queensland, is said to be taking unprecedented security measures for the summit, with some 6,000 police deployed.
Twenty-seven different groups have been given permits to protest at designated areas near the Brisbane Convention and Exhibition Centre, the venue hosting the meeting, and thousands of people are expected over the weekend.
More than 200 protesters buried their heads in the sands of Bondi Beach on Thursday in a demonstration over climate change inaction.
The Australian government is facing criticism over its climate policies. Since coming to power, Mr Abbott has axed a carbon tax and overseena 70% fall in investment in renewable energy.
Eyes will also be on President Putin, amid tensions over Russia's role in Ukraine. Some Australians had called for Mr Putin to be blocked from the summit over the downing of flight MH17.
Thirty-eight Australians were among the 298 people who died when the plane was brought down over rebel-held eastern Ukraine, by a missile that the West believes was supplied by Russia. Russia rejects the claim.
Mr Abbott, who discussed the issue with Mr Putin in Beijing earlier this week, described Russia's actions in Ukraine as part of a "regrettable pattern" by a nation that was being "much more assertive".
He said he told Mr Putin that Russia should aspire to be a superpower "for peace and freedom and prosperity" instead of "trying to recreate the lost glories of tsarism or the old Soviet Union".
Meanwhile Mr Putin used a speech ahead of the G20 summit to warn that US and EU sanctions imposed over Russia's actions in Ukraine would harm not just Russia but the global economy.
What is G20?
- The "Group of Twenty" represents two-thirds of the world's population, 85% of global GDP and over 75% of global trade.
- It began in 1999 as an annual meeting for finance ministers and central bank governors following the Asian financial crisis, before evolving to also include a summit for countries' leaders in 2008, after the global financial crisis.
- G20 meetings are aimed at deepening economic co-operation and strengthening the global economy.
- It comprises 19 countries and the European Union.
- At each meeting, the host country invites non-member guest countries to attend. For 2014, Australia has invited Spain, Mauritania, Myanmar, Senegal, New Zealand and Singapore.
