G20: Obama warns of Asia 'intimidation' as summit begins
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".
Analysis: Jon Donnison, BBC News, Brisbane
President Obama's criticism of China was not explicit but reading between the lines of his speech, there was plenty the government in Beijing might take exception to. He praised Asean's effort to reach a code of conduct with China that reinforces international law in the South China Sea.
Warning that territorial disputes could spiral into confrontation, he said America had an iron clad commitment to the sovereignty of its allies. There was praise for China too, notably for reaching a climate change deal earlier this week and also for lifting millions out of poverty.
The much anticipated "pivot to Asia" that many thought would be a key theme of Barack Obama's presidency has never fully materialised. US involvement in a persistently volatile Middle East put paid to that.
But the American leader has used this visit to Australia, as well as this week's Apec summit in China and Asean meeting in Burma, to remind people that he sees US engagement in Asia as key in the 21st Century.
'Carry the economy'
President Obama did not mention China explicitly but he warned of the dangers posed by territorial disputes in the South China Sea, where Beijing's actions have raised concern among its neighbours.
He also said that all countries had to increase prosperity, warning that America could not "carry the global economy on its back".
Opening the summit, Australian Prime Minister Tony Abbott said he wanted to use the event to reassure people about the direction of the world's economy, with a "message of hope and optimism".
He said job creation, identifying tax cheats and strengthening the global economy would all be discussed.
His government had tried to keep climate change off the agenda, despite calls from campaigners.
More than 200 protesters buried their heads in the sands of Bondi Beach on Thursday in a demonstration over climate change inaction.
In other developments, G20 leaders released a statement in which they vowed to do all they could to "extinguish" the Ebola outbreak in West Africa.
It said that members state were committed to do what was necessary "to ensure the international effort can extinguish the outbreak and address its medium-term economic and humanitarian costs".
The summit is taking place amid tight security, with some 6,000 police deployed. Twenty-seven different groups have been given permits to protest at designated areas.
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.
Source: G20 website