According to an Ukrinform correspondent, he said this while commenting on the Kremlin's reaction to the UK's plans to supply Ukraine with ammunition that contains depleted uranium.
"This is a commonplace type of munition that is used typically for its armor-piercing capabilities. […] Russia doesn't want Ukraine to continue to take out its tanks and render them inoperable," Kirby said.
He said that depleted uranium munitions are not a radioactive threat.
"If Russia is deeply concerned about the welfare of their tanks and tank soldiers, the safest thing for them to do it move them across the border, get them out of Ukraine," Kirby said.