WASHINGTON (Reuters) - The United States does not believe that there is a threat of Russia using nuclear weapons despite a recent escalation in Moscow's rhetoric, a senior U.S. defense official said on Friday.

"We continue to monitor their nuclear capabilities every day the best we can and we do not assess that there is a threat of the use of nuclear weapons and no threat to NATO territory," the official, speaking on the condition of anonymity, told reporters.

Russian Foreign Minister Sergei Lavrov said on Monday the West should not underestimate the elevated risks of nuclear conflict over Ukraine.

Russia said earlier this month that it plans to deploy its newly tested Sarmat intercontinental ballistic missiles, capable of mounting nuclear strikes against the United States, by autumn.

Western concern at the risk of nuclear war increased after President Vladimir Putin launched Russia's invasion of Ukraine on February 24 with a speech in which he pointedly referred to Moscow's nuclear forces and warned that any attempt to get in Russia's way "will lead you to such consequences that you have never encountered in your history."

British Prime Minister Boris Johnson said this week he did not expect any further Russian military failures in Ukraine to push Putin into using tactical nuclear weapons there, saying the Russian leader had room to maneuver and end the conflict.

Earlier this month CIA Director William Burns said the threat of Russia potentially using tactical or low-yield nuclear weapons in Ukraine could not be taken lightly, but that the CIA has not seen a lot of practical evidence reinforcing that concern.

Russia's invasion of Ukraine has left thousands dead or injured, reduced towns and cities to rubble and forced over 5 million people to flee abroad. Moscow calls its action a "special operation" to demilitarize its neighbor.

(Reporting by Idrees Ali and Phil Stewart; diting by Frances Kerry)