Prime Minister Tony Abbott has warned Russian President Vladimir Putin to "back off" from invading neighbouring Ukraine.
On Saturday Putin won parliamentary approval to send troops into the Ukraine - a direct rebuff to Western leaders, including US President Barack Obama, who repeatedly urged Russia not to intervene. Unmarked troops – some in vehicles with Russian number plates - have already seized Crimea, an isolated peninsula on the Black Sea.
"This is very, very concerning and I think that every Australian, and people right around the world, will be thinking: hands off the Ukraine," Mr Abbott told Channel Ten's Bolt Report on Sunday morning. "This is not the kind of action of a friend and neighbour and I think Russia should back off."
Mr Abbott said he understands the issue will be discussed "urgently" by the members of the United Nations Security Council, including Australia.
"Plainly, this is unacceptable behaviour," Mr Abbott said. "You cannot just cross the border of another country with military force, with aggressive intent."
Mr Abbott would not comment on whether he would support the US intervening to stop any Russian aggression.
Ukrainian President Viktor Yanukovych fled to Russia in February after the Ukrainian Parliament voted to oust him.
On Saturday night, US time, the White House issued a statement saying Mr Obama had told Mr Putin in a 90-minute phone call that Russia's continued violation of international law will lead to greater political and economic isolation.
The US has already announced it will suspend upcoming participation in preparatory meetings for the G-8.
"President Obama expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of international law," the White House statement said.
"President Obama made clear that Russia's continued violation of Ukraine's sovereignty and territorial integrity would negatively impact Russia's standing in the international community."
Mr Putin's office issued a statement on the call saying: "In response to the concern shown by Obama about the plans for the possible use of Russia's armed forces on the territory of Ukraine, Putin drew attention to the provocative, criminal actions by ultra-nationalists, in essence encouraged by the current authorities in Kiev.
"The Russian President underlined that there are real threats to the life and health of Russian citizens and compatriots on Ukrainian territory. Vladimir Putin stressed that if violence spread further in the eastern regions of Ukraine and in Crimea, Russia reserves the right to protect its interests and those of Russian-speakers living there."
The UN Security Council will hold an emergency meeting on Sunday, reportedly at the request of the United Kingdom.