The US president says Asia’s security must not be based on intimidation of small nations by big ones.
Barack 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 his commitment to Asia-Pacific allies, referring to US efforts to strengthen strategic ties with the region.
The two-day G20 summit will focus on promoting economic growth.
World leaders are expected to expand on plans agreed by G20 finance ministers in February to boost global growth by 2% in five years.
UN Secretary General Ban Ki-moon also urged those attending to meet the challenges posed by Ebola, climate change and the conflict in Ukraine.
‘Get out of Ukraine’
Meeting on the margins of the summit, Russian President Vladimir Putin urged his French counterpart Francois Hollande to join him to defuse tensions between the two countries over Ukraine.
France has withheld the delivery of two warships to the Russian navy over its actions in Ukraine.
The Russian president also discussed “rebuilding relations” with the UK after a bilateral meeting with Prime Minister David Cameron, Mr Putin’s spokesman Dmitry Peskov said.
Before the summit began, Mr Cameron said Russia could face further sanctions unless it stopped “destabilising Ukraine”.
But Mr Putin faced a frosty reception from Canadian Prime Minister Stephen Harper over Russia’s military assertiveness in Ukraine.
“I’ll shake your hand, but I only have one thing to say to you: you need to get out of Ukraine,” Mr Harper told the Russian leader.
Earlier President Obama said Russia’s “aggression” towards Ukraine was a “threat to the world”.