While there, the official said, Burns “discussed the US warning he delivered to the head of Russia’s SVR not to use nuclear weapons and reinforced the US commitment to provide support to Ukraine in its fight against Russian aggression.”

The flurry of back-channel communications comes less than a week after Russia announced a withdrawal from a key Ukrainian city and a quiet debate has begun in Washington over whether or not to encourage Ukraine to pursue a diplomatic resolution to the war. It also comes as the US has grown increasingly concerned that Russia could turn to a nuclear weapon in its struggling attack.

Burns and other US officials have said publicly that they see no evidence that Moscow is actively preparing to take such a step, but officials familiar with the intelligence warn that the risk is perhaps the highest it has been since Russia invaded Ukraine in February.

President Joe Biden has leaned heavily on Burns, an experienced diplomat with deep experience in Russia, as a quiet messenger in the ongoing conflict.

Burns was sent to Ankara on Monday to “communicate with Russia on managing risk, especially nuclear risk and risks to strategic stability,” a national security spokesman said. The spokesman emphasized that he did not conduct negotiations of any kind.

In Kyiv in October, he “reinforced the US commitment to provide support to Ukraine in its fight against Russian aggression, including continued intelligence sharing,” a US official told CNN at the time.

Burns was sent to Moscow last November, before Russia invaded Ukraine, to warn the Kremlin of the consequences of an invasion. He has also been involved in discussions with Naryshkin about US citizens detained in Russia, including Brittney Griner and Paul Whelan.