WATCH: White House press secretary Karine Jean-Pierre holds news briefing

White House press secretary Karine Jean-Pierre will hold a news briefing Tuesday amid growing fears of a Russian “dirty bomb.”

TWatch the briefing in the player above.

Ukraine’s nuclear energy operator said Tuesday that Russian forces were performing secret work at Europe’s largest nuclear power plant, activity that could shed light on Russia’s claims that Kyiv’s forces are preparing a “provocation” involving a radioactive device.

WATCH: Defense Secretary Austin calls out Russia’s ‘nuclear saber rattling’

Russian Defense Minister Sergei Shoigu made an unsubstantiated allegation that Ukraine was preparing to launch a so-called dirty bomb. Shoigu made the charge in calls to his British, French, Turkish and U.S. counterparts over the weekend. Britain, France, and the United States rejected it out of hand as “transparently false.”

The briefing also follows a Russian court rejecting Britteny Griner’s appeal to her 9-year sentence.