Brisbane, Nov 15 (IANS): Security in the Asia-Pacific region must depend on international law and the peaceful resolution of disputes, not on "intimidation", US President Barack Obama said Saturday ahead of the G20 summit here, media reported.

Addressing students in Brisbane, Obama emphasised that Asia's security must be based on mutual alliances and big nations must not bully smaller ones, according to a BBC report.

"We believe... that any effective security order for Asia must be based not on spheres of influence, coercion or intimidation, where big nations bully the small, but on alliances for mutual security, international law and... the peaceful resolution of disputes," Obama was quoted as saying.

He warned of the dangers posed by territorial disputes in the South China Sea, where Beijing's perceived assertiveness has raised concern among its neighbours.

There was "no question" over US's commitment to its Asia-Pacific allies, Obama added.

President Obama said Russia's "aggression" towards Ukraine was a "threat to the world", citing the downing of the Malaysia Airlines flight MH17 last July.

Putin was earlier attacked by both Australian Prime Minister Tony Abbott and British Prime Minister David Cameron over what the West believes to be Russian complicity in the downing of the flight.

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. However, Russia stridently rejects the claim.

Abbott, who discussed the issue with Putin in Beijing earlier this week, described Russia's actions in Ukraine as part of a "regrettable pattern" and that Russia should aspire to be a superpower "for peace, freedom and prosperity" instead of "trying to recreate the lost glories of tsarism or the old Soviet Union".

Obama used this speech to also announce that the US would contribute $3 billion to an international fund to help poor countries cope with the effects of climate change.