The relatively rare Russia-US contact came during the mutiny by Yevgeny Prigozhin's Wagner group, which briefly took control of a major city and threatened an attack on Moscow. The upheaval sparked fears in Washington that nuclear-armed Russia could descend into chaos.

"Instability in Russia is something that, you know, we take seriously and we certainly had lots of questions over the course of the weekend," National Security Council spokesman John Kirby said, when asked about fears over turmoil in nuclear-armed Russia.

"We did have and were able to have in real time -- through diplomatic channels -- conversations with Russian officials about our concerns," he said.

Kirby said US officials continue to monitor Moscow's nuclear posture "very closely" during the invasion by Russian troops of pro-Western Ukraine.

So far there is "no indication" that Russian President Vladimir Putin is moving in the "direction" of using nuclear weapons or "anything that will cause us to change our own deterrent posture," Kirby said.