BRISBANE Australia - Western leaders and Russian President Vladimir Putin, following fresh reports of Russian troops pouring into eastern Ukraine.
Ukraine has accused Russia of sending soldiers and weapons to help separatist rebels in eastern Ukraine launch a new offensive in a conflict that has killed more than 4,000 people.
British Prime Minister David Cameron blasted Russia's actions as unacceptable on Friday, warning that they could draw greater sanctions from the United States and the European Union.
"I would still hope that the Russians will see sense and recognise that they should allow Ukraine to develop as an independent and free country, free to make its choices," Cameron told reporters in Canberra.
"If Russia takes a positive approach towards Ukraine's freedom and responsibility, we could see those sanctions removed, if Russia continues to make matters worse then we could see those sanctions increased, it's as simple as that."
Russia denies sending troops and tanks into Ukraine.
But increasing violence, truce violations and reports of unmarked armed convoys traveling from the direction of the Russian border have aroused fears that a shaky Sept. 5 truce could collapse.
The G20 leaders summit in Brisbane is focused on boosting world growth, fireproofing the global banking system and closing tax loopholes for giant multinationals.
But with much of the economic agenda agreed and a climate change deal signed last week in Beijing between the United States and China, security concerns are moving to centre-stage.
Ukraine has not been a top focus during a pair of summits in Asia this past week, US Deputy National Security Advisor Ben Rhodes said, although President Barack Obama did raise it briefly with Putin when both attended the Asia Pacific Economic Cooperation forum in China.
Obama arrives in Brisbane on Saturday and will be discussing his frustration over Ukraine with a key bloc including German Chancellor Angela Merkel, French President Francois Hollande and Cameron.
"They've been key towards sending a shared message to the Russians and the Ukrainian government," Rhodes told reporters. "So it will be an opportunity for him to check in with them."