Barack Obama, the US president, has said the annexation of Crimea from Ukraine is not a "done deal" and described Russia as "a regional power" that was threatening its neighbours "out of weakness".
Obama told journalists after a nuclear security summit in The Hague on Tuesday that "the fact that Russia felt to go in militarily and lay bare these violations of international law indicates less influence, not more".
"We [the United States] have considerable influence on our neighbours," he said. "We generally don't need to invade them in order to have a strong cooperative relationship with them."
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Obama rejected Russian President Vladimir Putin's claim that Russian speakers had been threatened in Crimea and in other Ukrainian territories and claimed analogies to Kosovo made "absolutely no sense".
"When I hear analogies to Kosovo, where you had thousands of people who were being slaughtered by their government, it's a comparison that makes absolutely no sense," he said.
"I think it is important for everybody to be clear and strip away some of the possible excuses for potential Russian action."
The president said the build-up of Russian troops along the Ukrainian border appears to be an attempt to intimidate Ukraine, but the troops were on Russian soil and it was Moscow's right to have troops there.
Obama's comments came a day after the Group of Seven industrialised countries, that includes the US, discussed how to pressure the Kremlin diplomatically for its actions in Crimea and discourage Moscow from further military actions in Ukraine.
"We remain ready to intensify actions including coordinated sectoral sanctions that will have an increasingly significant impact on the Russian economy, if Russia continues to escalate this situation," they said in a joint statement.
The G7 leaders, who met on the sidelines of a nuclear security summit in The Hague, said they would convene again in Brussels in early June, the first time since Russia joined the G8 in 1998 that it will have been shut out of the annual summit of industrialised democracies.
They also urged the International Monetary Fund to reach a rapid agreement with Ukraine to unlock urgently needed financial aid for the country's shattered economy.