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.
But the president also said that the United States had not yet determined whether Russia’s assertions that Kyiv was planning to set off a dirty bomb were intended to create a pretext for escalating its own attacks.
Mr. Biden’s comments came after three days of Russian accusations, made without providing evidence, that Ukraine was making dirty bombs at two facilities. Both facilities are regularly inspected by the International Atomic Energy Agency, the United Nations’ nuclear watchdog.