Why do you think not?
too big and too far for the gmlrs rockets and the u.s. has categorically stated that they haven't provided tacms rockets. the united states has provided some stuff and not announced it till after it was used, but they (afaik) haven't provided anything that they've denied providing and i don't see any reason that they would burn credibility to do so in this case.
also, with previous u.s. rockets, russia has recovered fragments and released footage of them pretty much as soon as they entered use, presumably because it helps maintain domestic support for the war. they haven't done that here.
ukraine does have the neptune missile with adequate range, and i think i've seen talk that they've been working on another long-range missile. i've seen speculation that they may have repurposed/developed one of those to fire from an aircraft, possibly with foreign assistance in upgrading targeting accuracy, and that makes more sense to me than a weapon that both ukraine and the u.s. deny that they have. i've also seen speculation about having used repurposed u.s. made anti-radiation harm missiles, but that also runs into the u.s. denial that u.s. made weaponry was used, and i'm also not sure they'd produce an explosion that large.
i imagine we'll find out eventually.
