Viktor Drachev/AFP
Armed men who call themselves members of Ukraine's disbanded elite Berkut riot police force stand at a checkpoint on a highway that connects the Black Sea Crimea peninsula to mainland Ukraine near the city of Armyansk, on February 28, 2014
"It looks like they've moved several hundred troops there (into Crimea)," a senior defense official, who spoke on condition of anonymity, told AFP.
The comments marked the first confirmation from President Barack Obama's administration that Russia had launched an incursion into Crimea.
The Russians had not given the US government advance notice of the action or explained its intentions since the operation was launched, officials said.
But Pentagon officials stressed Washington was focused on diplomacy, and there was no serious consideration being given to any US military action.
"It is now in the realm of diplomacy," said a second defense official, who asked not to be named.
Officials also said they were not aware of any request from Ukraine for military assistance.
