The US has distanced itself from an incursion into Russia - which Moscow says ended in the defeat of armed insurgents who entered from Ukraine.
Parts of the border region of Belgorod came under attack on Monday, in one of the largest cross-border raids since Russia invaded its neighbour last year.
A state department spokesman acknowledged reports "circulating on social media and elsewhere" that US-supplied weapons had been used, but said his country was "sceptical at this time of the veracity of these reports".
Villages in Belgorod near the border were evacuated after coming under fire. Russia says 70 attackers were killed, and has insisted the fighters were Ukrainian.
But Kyiv denies involvement - and two Russian paramilitary groups opposed to Russian President Vladimir Putin say they were behind the incursion.
Monday's raid led Moscow to declare a counter-terrorism operation, giving the authorities special powers to clamp down on communications and people's movements.