The US has distanced itself from the incursion into Russia – which Moscow says ended with the defeat of armed rebels who entered Ukraine.
Parts of the Belgorod border region came under attack on Monday, one of the largest cross-border attacks since Russia invaded the neighboring country last year.
Russia later released photos of abandoned or destroyed Western military vehicles, including US-made Humvees.
The US insisted it “did not encourage or enable attacks inside Russia”.
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”.
In a news briefing on Tuesday, Matthew Miller added: “It’s up to Ukraine how they conduct this war.”
The villages of Belgorod near the border were evacuated after being hit by fire. Russia says 70 attackers were killed, and insists the fighters were Ukrainian.
But Kyiv has denied involvement – and two Russian paramilitary groups opposed to Russian President Vladimir Putin say they were behind the infiltration.
As a result of Monday’s raid, Moscow announced an anti-terror operation, giving authorities special powers to block communications and the movement of people.
These measures were taken the following afternoon, and even then, one of the paramilitary groups was claiming that it still controlled a small piece of Russian territory.
Belgorod’s governor said one civilian died and several others were injured during the violence.
In a later development, Vyacheslav Gladkov said on Tuesday evening that Belgorod came under fresh attack from a drone that dropped an explosive device and damaged a car.
He said the flying vehicle had been shot down and there appeared to be no new casualties.
The claims by the warring parties have not been independently verified – although JEE News was able to establish that a building used by Russia’s main security agency, the FSB, was among those affected during the violence. It is unclear what caused the damage.
