The official, on condition of anonymity, on Friday said that the trip to Russia could be cancelled as part of the "costs" that Obama earlier posited if Moscow is judged to infringe Ukrainian sovereignty as a geopolitical crisis over the former Soviet state threatens to boil over.

Other European leaders were also considering whether they will show up at the summit, which marks a moment of great prestige for Russian President Vladimir Putin, in the same Black Sea resort that recently hosted the Winter Olympics.

Last year, Obama cancelled a head-to-head summit with Putin but did show up at the G20 summit in St Petersburg as relations between Moscow and Washington plummeted over the Russian offer of asylum to fugitive US intelligence leaker Edward Snowden.

The official confided a list of possible US reprisals after Obama appeared in the White House press briefing room to warn he was deeply concerned at reports that Russia had moved more troops into the Crimea, where it already has a military base.
    
Another US official later said on condition of anonymity that Washington believed several hundred Russian forces were involved.
    
Obama is also mulling a set of economic steps that he could take against Russia, the first senior official said, noting that a team of Russian officials was in Washington as recently as this week seeking to deepen trading and commercial ties.
    
The official also noted that Russia would lose any moral or goodwill dividend that it had amassed during the Olympics, which ended last weekend, if it was seen as infringing the sovereignty of Ukraine.

(Agencies)

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