Do we have some kind of source to back this up?
Here's Spokesperson Matthew Miller at the Department of State daily press briefing, May 22:
"...]
QUESTION: So, in Belgorod there were – there are obviously these reports, and there’s images online that suggest American weapons have been used. So, I know you don’t know, but do you support U.S. weapons being used on Russian territory? [...]
MR MILLER: [...] And then with respect to the broader policy question, we have made very clear to the Ukrainians that
we don’t enable or encourage attacks outside Ukrainians’ borders, but I do think it’s important to take a step back and remind everyone, and remind the world, that it – of course it is Russia that launched this war. It’s Russia that continues to launch attacks on civilians in Ukraine. It’s Russia that’s targeted schools and hospitals and civilian infrastructure.
So, it is up to Ukraine to decide how they want to conduct their military operations, but it is Russia that has been the aggressor in this war. [My underline.] [..."
1:31 p.m. EDT MR MILLER: Hello, everyone. Sorry for being a few minutes late. QUESTION: Yes, well, we won’t hold it against you, your first day on the job and you show up 15 minutes — MR MILLER: Are you going to let me speak, before we get into grilling me, Matt? QUESTION: Of course. […]
www.state.gov
And here in motion picture. Fast forward to the 26:05 mark: