A Ukrainian official is warning Russian forces are preparing for “the heaviest of battles” in the strategic southern region of Kherson as Ukrainian troops continue their advance on the occupied area. Thousands of residents are still evacuating. On Tuesday, President Biden warned Russia against using nuclear or radioactive weapons in Ukraine following Moscow’s unsupported allegation that Ukrainians would deploy a “dirty bomb.” The U.S., along with Kyiv and European allies, have rejected Russia’s accusation.

President Joe Biden: “Russia would be making an incredibly serious mistake if it were to use a tactical nuclear weapon. I’m not guaranteeing you that it’s a false flag operation yet, don’t know, but it would be a serious, serious mistake.”

The White House said Tuesday they have no evidence Russia is preparing to use such weapons. Meanwhile, Ukraine’s nuclear energy operator said Russian forces occupying the Zaporizhzhia nuclear plant could be “preparing a terrorist act using nuclear materials and radioactive waste stored.”