Russia's Cold War-style standoff with the West sent a chill through G20 talks aimed at heating up the world economy on Saturday as leaders kicked off a summit in Australia.
Global warming also emerged as a focus for leaders including Barack Obama, Russia's Vladimir Putin and Xi Jinping of China, posing another distraction for the Australian hosts after they had committed to confining the Brisbane summit to an ambitious growth agenda.
Australian Prime Minister Tony Abbott, a sceptic on man-made climate change, came into the summit locked in a war of words with Putin over the downing of a Malaysia Airlines passenger jet in July.
Australian anger at Putin was underlined by the Saturday edition of Brisbane's Courier-Mail newspaper. Its front page showed an Australian kangaroo boxing a growling Russian bear, over the headline "Ice Cold War."
While the Ukraine crisis does not figure in the formal G20 deliberations, tackling climate change and Ebola were reportedly to feature in the leaders' final declaration on Sunday.
Obama was due to announce Saturday a $3 billion pledge to a UN fund aimed at mitigating the effects of climate change in the world's poorest countries, after emerging from a pre-G20 trip to China with a pact with Xi on greenhouse gas emissions.
The US president's renewed focus on global warming has enthralled scientists and campaigners but infuriated his Republican opponents, who are retaking control of Congress after this month's mid-term elections.
It also threatens to disquiet Abbott, who wants the G20 instead to deliver on its commitment of lifting economic growth by up to two trillion dollars in the coming years.
If the pledge is delivered, that will deliver a "message of hope and aspiration to the world", Australian Treasurer Joe Hockey said Saturday.
The idea in Brisbane is to flesh out the growth plan -- and also to close corporate loopholes that allow some multinational companies to pay a pittance in tax depending on where they are domiciled after a major dispute erupted over Luxembourg's beneficial tax deals with a slew of companies.
The prime minister of Luxembourg at the time was Jean-Claude Juncker, who is now the head of the European Union's executive commission and finds himself under a hail of rhetorical fire.
In Brisbane on Saturday Juncker defended his position over the sweetheart deals extended when he led the wealthy duchy, and endorsed a global fight against tax evasion in the works from the G20.
"In accordance with the law you can create a situation, the result of it is a very low taxation for companies. This has to be avoided," Juncker told reporters.
- 'Tsarist ambitions' -
Further afield, the G20 is under pressure to adopt a hard-hitting financial response to the Ebola epidemic in west Africa as health workers battling horrific working conditions plead for more resources.
A joint petition from aid groups including Oxfam and Save the Children urged the G20 to band together to ensure the right resources are made available in terms of personnel, equipment and funding.
"This is a chance to stop Ebola in its tracks, and it must not be missed," said Oxfam Australia chief Helen Szoke.
Yet while the world's most powerful countries stake out common ground on the economy and on fighting disease, familiar strains on the geopolitical front have again erupted into the open.
Putin arrived in Brisbane facing an icy reception after the prime ministers of Britain and Australia accused him of being a bully and harbouring imperialist ambitions in the context of the Ukraine crisis.
Moscow hit back with strong language against the West that included a warning for France against further delay in handing over a warship promised under a 2011 contract, as Putin and French President Francois Hollande prepared to hold bilateral talks Saturday evening in Brisbane.
Putin is also due to meet German Chancellor Angela Merkel at the summit, and started Saturday with a separate meeting of the BRICS countries -- Brazil, Russia, India, China and South Africa.
Interviewed by the TASS news agency, the Russian strongman said his government was prepared to deal with a potentially "catastrophic" fall in oil prices.
Their decline to four-year lows is a boon for growth in other G20 countries but threatens to explode Russia's deficit and is already depressing the ruble.
An overhaul of the global energy market could be a surprise outcome of the G20 summit with plans for a new agency to protect against oil and gas supplies being used as foreign policy tools, The Australian newspaper said.
Putin also assailed other G20 countries for imposing sanctions over Ukraine and the Malaysia Airlines plane incident.
But he said he would not raise the topic of sanctions at the G20. "Why should I draw attention to this, ask for something? It's pointless."