WASHIGTON - The United States’ military is keeping a constant watch on Russia’s nuclear arsenal as the war in Ukraine continues.

Defense Secretary Lloyd Austin is being briefed two or three times a week by the top US general who oversees US nuclear weapons and defenses, according to multiple defense officials.

The US has not seen any indication Russia has made any moves to prepare nuclear weapons for use during the war, but two sources familiar with recent intelligence assessments told CNN that US officials are more concerned about the threat of Russia using them than at any time since the Cold War.

The sources stressed, however, that it is still unlikely Russian President Vladimir Putin would use any kind of nuclear weapon and one of the sources put the chances of use at around 1%.

Adm. Charles Richard, head of the US Strategic Command, is providing Austin and other top Pentagon leaders with a highly classified operation and intelligence update on the status of Russia’s arsenal and any moves that might cause concern, according to the defense officials.

It includes input from the intelligence community which closely monitors statements from Putin and other senior Russian leaders. The officials emphasized that if there are any sudden developments in between scheduled meetings, Austin and Joint Chiefs Chairman Gen. Mark Milley would be quickly briefed before updating the White House.

Monitoring any possible nuclear activity has always been a high priority for the Pentagon but the urgency of efforts increased shortly after Russia launched its invasion in February when Putin put the country’s deterrence forces, including Moscow’s nuclear weapons, onto the highest state of alert.

On Tuesday, Russia Foreign Minister Sergey Lavrov pushed back on a warning from Ukrainian President Volodymyr Zelensky in a CNN interview last week that Russia could be prepared to use nuclear weapons in Ukraine, stressing that Russia historically has been against the use of such weapons.