Ukraine crisis: Russia under pressure at G20 summit
Russia has been rebuked by Western leaders about its role in the Ukraine crisis, at a G20 summit in Australia.
Canadian Prime Minister Stephen Harper told Russian President Vladimir Putin that he needed to "get out of Ukraine".
US President Barack Obama said Moscow's "aggression" in Ukraine was a "threat to the world", while the UK threatened more sanctions unless Russia stopped "destabilising" its neighbour.
The two-day summit in Brisbane is focusing on promoting economic growth.
World leaders are expected to elaborate on plans agreed by G20 finance ministers in February to boost global growth by 2% in five years.
However, Saturday - the first of the two-day summit - was dominated by Ukraine, where pro-Russian separatists have been fighting government forces in eastern regions.
Ukraine and its Western allies have accused Russia of sending military forces across the border, something the Kremlin denies.
The EU imposed sanctions when Russia annexed Crimea in March and has added further measures since.
Before the G20 summit began, UK Prime Minister David Cameron said there would "have to be a very different relationship" between Europe and Russia if "we continue to see Russian troops" inside Ukraine, adding that there was "the potential for further sanctions".
Mr Cameron later held a private meeting with Mr Putin. The two discussed "rebuilding relations", Mr Putin's spokesman Dmitry Peskov said.
The Russian president faced a frosty reception from Canada's Mr Harper.
"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 him, the Canadian leader's spokesman said.
President Obama said the US was at the forefront of "opposing Russia's aggression against Ukraine, which is a threat to the world".
Analysis: Gary O'Donoghue, BBC News
If President Putin expected a warm diplomatic welcome at the G20 under the blazing Brisbane sun, he was disappointed; he was in fact subjected to something more akin to a severe Siberian winter.
The West believes Russia is behind the escalation of tension in eastern Ukraine - a conflict that has already cost 4,000 lives. Tensions have also been increased by Russian military activity around Nato's borders, with several instances of fighter planes being scrambled to intercept Russian aircraft.
On Monday, EU foreign ministers will consider whether to extend sanctions against Russia - there are already restrictions on the energy, defence and finance sectors as well as travel bans and asset freezes on individuals.
President Putin denies he is directly involved in Ukraine, and dismisses sanctions as pointless. It all amounts to a standoff between the West and Russia - and little sign of any thaw in prospect.
The BBC's Jon Donnison in Brisbane says President Putin cut something of an isolated figure among the world's powerbrokers.
Reports quoted Russian officials as saying Mr Putin was planning to leave the summit ahead of schedule on Sunday, but gave no reasons for the move.
However, the president's spokesman Dmitry Peskov denied this, saying that Mr Putin would only leave "when all the work is completed."
Meeting on the margins of the summit on Saturday, Mr 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.
'Message of hope'
Meanwhile, President Obama said Asia's security must not be based on intimidation of small nations by big ones, but on mutual alliances.
He 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.
Mr Obama said there was "no question" over his commitment to Asia-Pacific allies, referring to US efforts to strengthen strategic ties with the region.
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.
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".
- World leaders and their spouses were given koalas to hold on the fringes of the summit - the animals are native to Australia
- First protected by law in the 1930s after being hunted to near extinction by fur traders in parts of Australia; declared a threatened species in 2012
- Species recently severely affected by chlamydia, which can cause blindness, infertility and death among the animals
- Began being used as a diplomatic tool in early 1980s, after government lifts export ban - they are often given to foreign zoos as gifts
- They are not bears, but are marsupials