Russia-Ukraine updates: Putin suspends key US-Russia nuclear treaty in speech denouncing West

President Joe Biden told reporters he is unsure if disputed Russian claims that Ukraine's military is planning to use a "dirty bomb" were a "false-flag operation" or if Russia is planning on deploying a dirty bomb itself, warning Russia against using nuclear weapons.

"Russia would be making an incredibly serious mistake for it to use a tactical nuclear weapon. I'm not guaranteeing you that it’s a false flag operation yet, I don’t know, but it would be a serious, serious mistake," Biden told reporters at the White House on Tuesday.

-ABC News' Ben Gittleson

Energoatom, Ukraine’s state nuclear energy operator, accused Russian forces of performing secret construction work at the occupied Zaporizhzhia nuclear power plant over the last weeks, amid allegations from Russia that Ukraine's military is preparing a "provocation" involving a radioactive device.

In calls with his British, French, Turkish and American counterparts over the weekend, Russian Defense Minister Sergei Shoigu made an unsubstantiated allegation that Ukraine was preparing to launch a so-called dirty bomb. Britain, France and the U.S. rejected the claims calling them "transparently false."

Ukraine also dismissed Moscow’s claim as an attempt to distract attention from the Kremlin’s own alleged plans to detonate a dirty bomb, which uses explosives to scatter radioactive waste in an effort to sow terror.

-ABC News' Will Gretsky

U.S. Secretary of State Antony Blinken spoke with his Ukrainian counterpart, Foreign Minister Dmytro Kuleba, on Monday after having a call with him on Sunday, and the "rhetoric surrounding so-called dirty bombs" was again on the agenda.

"The secretary reaffirmed enduring U.S. support for Ukraine in the face of continued Russian aggression, atrocities and rhetoric surrounding so-called 'dirty bombs' in Ukraine," State Department spokesperson Ned Price said in a statement. "He noted our commitment to work with allies and partners to continue meeting Ukraine's security assistance needs on the battlefield."

Blinken tweeted: "Connected with @DmytroKuleba again today. It is important to once again emphasize that U.S. support for Ukraine is concrete, comprehensive and enduring."

-ABC News' Shannon Crawford

A senior commander of the Russian military said Monday that his troops have been readied to operate "in the conditions of radioactive contamination," amid Moscow's allegation that Ukraine is preparing to use a "dirty bomb" on its own territory.

"Work has been organised by the [Russian] Ministry of Defense for combating possible provocations from the side of Ukraine: forces and equipment have been put in readiness for fulfilling tasks in the conditions of radiative contamination," Kirillov said during a press briefing, as quoted by Russian state media.

The comments are further worrying signs that Russia is trying to build a false-flag narrative, blaming Ukraine for the possible use of nuclear weapons, which is clearly intended as a threat to both Ukraine and its Western allies.

-ABC News’ Patrick Reevell