President Barack Obama has labelled Russia a "regional power" that is threatening its neighbours out of weakness, not strength - but admitted further encroachment into Ukraine was an increasing concern.
Speaking at a summit of the G7 industrial leaders in The Hague, the US president reiterated the West was ready to crank up sanctions against Russia should it continue to aggravate the crisis in Crimea and Ukraine.
Mr Obama said: "Russia is a regional power that is threatening some of its immediate neighbours, not out of strength but out of weakness.
"We (the US) generally don't need to invade our neighbours in order to have a strong co-operative relationship with them.
"The fact that Russia felt the need to go in militarily and lay bare these violations of international law indicates less influence, not more."
Mr Obama said Russia's annexation of Crimea was not a "done deal" as it has not been recognised by the international community.
"It is up to Russia to act responsibly and show itself once again to be willing to abide by international norms, and if it fails to do so there will be some costs," he said.
Mr Obama also rejected President Vladimir Putin's claim that Russian speakers had been threatened in Crimea and Ukraine.
"I think it is important for everybody to be clear and strip away some of the possible excuses for potential Russian action," he said.
The Cold War-style showdown dominated international talks in The Hague for a second day.
During a meeting with Chinese President Xi Jinping and UN Secretary-General Ban Ki-moon, David Cameron warned that Russia faced further diplomatic and political isolation if it intervened in eastern Ukraine.
Mr Cameron said: "What I think we need to do is just send the clearest possible message about the eastern Ukraine and the steps that we don't want to see Putin take.
"We need to send a very clear message that would lead to significant sanctions."
The Prime Minister's comments come a day after the G7 - the US, UK, France, Germany, Canada, Italy and Japan - agreed to pull out of a scheduled G8 meeting in Russia and hold its own meeting in Brussels.
The move effectively suspended two decades of cooperation with Moscow in the G8 "until Russia changes course".
In a joint statement, the G7 leaders said: "This group came together because of shared beliefs and shared responsibilities. Russia's actions in recent weeks are not consistent with them."
The US and European Union have already targeted some Russian individuals, but are yet to impose broader economic sanctions.
Foreign Secretary William Hague acknowledged that measures targeting Russia's economy could carry a negative impact in Europe, but said "we have to be prepared to do that".
"Every country would have to do what is necessary if more far-reaching sanctions were applied, accepting that it would affect different economies in different ways," he said.
"The United Kingdom is fully prepared to play its full part."
Russian representatives, also in the Hague, largely shrugged off the G7's comments.
Russia's Foreign Minister, Sergei Lavrov, said the G8 had been useful to discuss issues such as the nuclear tensions with Iran and the civil war in Syria, but that it was not a vital tool.
"If our Western partners believe the format has exhausted itself, we don't cling to this format," Mr Lavrov said.
"We don't believe it will be a big problem if it doesn't convene."
Mr Lavrov held his own meeting with his Ukrainian counterpart Andriy Deshchytsia on the sidelines of the nuclear talks.
Speaking ahead of the talks, Mr Deshchytsia warned that Ukraine would never give up Crimea.
He also warned Russia against making further attempts to seize Ukrainian territory amid reports that Russian troops are assembling along its borders.
Mr Deshchytsia told reporters: "Our military and civilians living in Eastern Ukraine, Ukrainians, Russians, other nationalities, they are ready to defend their homeland."
Earlier in the day, Ukraine announced it was pulling all its forces from Crimea after losing another military base to Russian troops in the early hours of the morning.
Russian forces have gradually seized control of the region since voters in Crimea agreed to join the Russian Federation in a swiftly arranged referendum following Mr Yanukovych's removal.
Since then, Nato's top commander in Europe, General Philip Breedlove, has warned that Russia's military force massing on Ukraine's border was "very, very sizeable and very, very ready".
He said he was worried the Russian military could make a move for Moldova's breakaway Transdniestria region.
Transdniestria, a narrow strip of land to Ukraine's southwest, already has a Russian military presence and most people there favour a union with the country.
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