Russia has accused Ukraine of using US-supplied rockets to shell civilian targets in the Russian Belgorod region.
Maria Zakharova, a spokeswoman for the Russian foreign ministry, claimed HIMARS rockets had been used and women and children had been killed.
Speaking at an economic forum in St Petersburg, Ms Zakharova said fragments of the rockets would serve as proof.
Sky News is unable to verify her assertions.
The Belgorod region is just over the border from Ukraine and close to Kharkiv, Ukraine's second city, which has been subjected to a Russian bombardment recently.
America recently gave permission for some of its weapons to be used on Russian territory - but only near Kharkiv.