He warned Russian President Vladimir Putin the international community "will be forced to apply costs" if Russia does not remove its troops from Crimea.

 

Earlier, leaders of the G7 group of nations issued a similar threat.

 

Ukraine PM Arseniy Yatsenyuk, speaking after meeting Mr Obama, said Ukraine "will never surrender" to Russia.

 

"It is absolutely unacceptable to have Russian boots on the Ukrainian ground in the 21st century, violating all international deals and treaties," he said.

 

The diplomatic appeals to Moscow come ahead of Sunday's referendum in Crimea, in which citizens will be asked if they want to stay with Ukraine or join Russia.

 

The Russian military and pro-Russian armed men moved in to seize key sites in Crimea - an autonomous² region of Ukraine whose population is mainly ethnic³ Russian - in late February after the fall of President Viktor Yanukovych.

 

 

President Obama said the US has "been very clear that we consider Russia's incursion into Crimea outside of its bases to be a violation⁴ of international law".

 

"We have been very firm in saying we will stand with Ukraine and the Ukrainian people in ensuring that territorial⁵ integrity and sovereignty is maintained," he added.

 

In reference to scheduled talks between US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov in London on Friday, Mr Obama said he hoped diplomatic efforts will result in a "rethinking of the process".

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