Brisbane: The G20 leaders summit in Australia starting on Saturday is setting up as a showdown between Western leaders and Russian President Vladimir Putin, following fresh reports of Russian troops pouring into eastern Ukraine.
Ukraine accused Russia on Thursday 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.
The United States has warned Russia the West might punish it further for its "military escalation" of the crisis.
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 momentum building on other big issues after a breakthrough climate change deal signed last week at APEC in Beijing between the United States and China, security concerns are moving 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 Obama did raise it briefly with Putin.
In Brisbane, Obama will be discussing his frustration over Ukraine with a key bloc including German Chancellor Angela Merkel, French President Francois Hollande and British Prime Minister David 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."
Increasing violence, truce violations and reports of unmarked armed convoys travelling from the direction of the Russian border have aroused fears that a shaky Sept. 5 truce could collapse.
Russia denies sending troops and tanks into Ukraine.
There had been calls from some in Australia to block Putin from attending the summit given Russia`s actions in Ukraine and the downing of Malaysia Airlines Flight MH17 by Russian-backed rebels, but the overwhelming consensus was against it.
Australia said this week that it was monitoring a deployment of Russian warships that had entered its waters.
"We`re seeing – regrettably – a great deal of Russian assertiveness right now in Ukraine. So, it’s not really surprising," Australian Prime Minister Tony Abbott said.
In addition to Ukraine, the crises in the Middle East are threatening to overshadow the economic agenda.
British nationals who become foreign fighters abroad could be prevented from returning home under new laws to deal with jihadists fighting in conflicts like Iraq and Syria, Cameron said in an address to the Australian parliament on Friday.
As host, Australia will continue pushing its growth agenda despite growing security tensions.
Canberra is pushing for an increase in global growth targets of 2 percent by 2018 to create millions of jobs and that goal appears on track. The 1,000 policy initiatives should add around 2.1 percent, the head of the OECD said.
Taxation arrangements of global companies such as Google Inc, Apple Inc and Amazon.com Inc have become a hot political topic following media and parliamentary investigations into how many companies reduce their tax bills.
The OECD has unveiled a series of measures that could stop companies from employing many commonly used practices to shift profits into low-tax centres.
Australian Treasurer Joe Hockey said Australia had won US cooperation to launch an "aggressive crackdown" on tax avoidance.
"I believe we have got the United States signed up," he said on Australian Broadcasting Corp radio. "They were cautious at first but obviously the United States itself has been missing out on revenue from a number of these large multinationals."