(CNN) Russia's defense minister accused Ukrainians of planning to use a so-called dirty bomb -- a claim that was strongly refuted by US officials on Sunday as a Russian false flag operation.
The allegation from Russian defense minister Sergei Shoigu came during a phone call with US Defense Secretary Lloyd Austin on Sunday, the second call in three days between the two top officials.
The Russian Ministry of Defense said the two discussed the situation in Ukraine but did not provide further details. It was Shoigu who initiated the phone call to Austin, according to a senior US administration official.
A second official familiar with the conversation said Shoigu made the claim about the planned usage of a dirty bomb, a weapon that combines conventional explosives and uranium. That claim, which the Kremlin has amplified in recent days, has been strongly refuted by the US, Ukraine and the United Kingdom as a Russian false flag operation.
Shoigu has made similar comments to his French and British counterparts as well.
"We reject reports of Minister Shoigu's transparently false allegations that Ukraine is preparing to use a dirty bomb on its own territory," National Security Council spokesperson Adrienne Watson told CNN in a statement. "The world would see through any attempt to use this allegation as a pretext for escalation."
The US is also watching very closely for any intelligence that Russia has a specific plan to blow up a major dam near Kherson where Russia has ordered citizens to evacuate, the official said.
Later Sunday, the US State Department released a joint statement with the foreign ministers of France and the UK that also called Shoigu's allegations false and reiterated their unified support for Ukraine's territorial sovereignty.
On Friday, Austin called Shoigu, the first call between the two in several months. Before Friday, the two had not spoken since May.