WASHINGTON — President Biden renewed his warning to President Vladimir V. Putin on Tuesday that it would be an “incredibly serious mistake” to use a tactical nuclear weapon in Ukraine, reflecting the increasingly urgent concern in Washington and among Western allies that Russia may be searching for a pretext to unleash such a weapon.
Mr. Biden said that he was still uncertain if Russia was trying to put together a “false flag operation” in which it would detonate a dirty bomb and blame the Ukrainians. A dirty bomb is not a nuclear weapon, but an improvised device that uses conventional explosives to spread radioactive material.