Washington CNN  — 

The consequences for Russia if it uses a nuclear weapon in its war on Ukraine have been conveyed to Russian President Vladimir Putin, US Secretary of State Antony Blinken said Wednesday.

“We’ve also communicated directly and very clearly to the Russians, President Putin about the consequences,” the top US diplomat said at a Bloomberg event. Blinken did not indicate how it was communicated to Putin or by whom, and principal deputy spokesperson Vedant Patel later suggested that US officials had not communicated directly with him.

“You have seen members of this administration dialogue directly with their counterparts in Russia and express these concerns and the potential for dire consequences,” which “no doubt have likely made its way to President Putin,” Patel said at a State Department briefing.

Biden administration officials have said that Moscow has been warned at the highest levels of the consequences for use of nuclear weapon in the war, but Blinken’s remark is the first explicit mention that the message has been communicated to Putin himself.

Blinken denounced Russia’s latest claim that Ukraine is considering the use of a “dirty bomb” as “another fabrication and something that is also the height of irresponsibility coming from a nuclear power.”

He said the United States has communicated directly with the Russians “about trying to use this false allegation as a pretext for any kind of escalation.”

“The reason this particular allegation gives us some concern is because Russia has a track record of projecting, which is to say, accusing others of doing something that they themselves have done or are thinking about doing,” Blinken said.

Blinken reiterated that the US is tracking the Kremlin’s nuclear saber-rattling “very carefully,” but hasn’t “seen any reason to change our nuclear posture.”

Despite Putin’s rhetoric, Russian Ambassador to the United Kingdom Andrey Kelin told CNN on Wednesday that Russia will not use nuclear weapons in its war against Ukraine.

“Russia is not going to use nukes. It is out of the question,” Kelin told CNN’s Christiane Amanpour.

However, actions taken by Moscow in recent weeks – the “dirty bomb” allegations, attacks on civilian infrastructure, looming defeats on the battlefield, and its annual military exercise – have increased concerns, a senior administration official said.

This official told CNN that the potential collapse of parts of Russia’s military in Ukraine could be the factor that could cause Putin to turn to nuclear weapon use. As such, the US is keeping a close eye on the developments in the Kherson region, where it’s not easy for Russian soldiers to retreat.

Russia informed the US of its annual GROM exercise, which includes its strategic nuclear forces, the Pentagon said. The Kremlin said in a statement Wednesday that Putin was leading military training drills involving practice launches of ballistic and cruise missiles.

The official said it may “sounds alarmist” to cite concerns about planned exercises but noted that they cannot be viewed in a silo: they do allow Russians to practice doing things like getting missiles into place and flying bombers to sea at a time when they are being pushed into a corner on the battlefield in Ukraine.

Despite these increased concerns, US officials have not seen evidence of Russian actions that would indicate Moscow is preparing to use nuclear weapons.

“We’ve seen no need to change our own nuclear posture. We don’t have any indication that Moscow is preparing to use nuclear weapons. But this type of rhetoric is concerning for many reasons,” State Department Spokesperson Ned Price said on Tuesday.

CNN’s Katharina Krebs contributed reporting.