Burns traveled to Kyiv Tuesday, one day after holding talks in Ankara with Russia's spy chief Sergei Naryshkin on the war and Moscow's threat to use tactical nuclear weapons to defend its interests.

That was the highest level face-to-face meeting of US and Russian officials since the beginning of the war, and Burns delivered a firm warning to Naryshkin "on the consequences of the use of nuclear weapons by Russia, and the risks of escalation to strategic stability," according to a White House Statement Monday.

Then Burns traveled to Ukraine to inform Zelensky and his Ukraine counterparts on the talks with Naryshkin, and to reiterate US support for Kyiv's war effort, the official told AFP on the basis of anonymity.

The statement Monday stressed that in his talks with Naryshkin, who heads Russia's SVR foreign intelligence service, Burns was "not conducting negotiations of any kind."

"He is not discussing settlement of the war in Ukraine," the statement said.

Burns was in Kyiv on a day when Russian forces launched a barrage of missiles striking targets across Ukraine, including the capital city.

But the official said Burns was safely inside the US embassy at the time, and has now departed.