The United States still has not seen any "muscle movements" following Russian President Vladimir Putin's weekend announcement that he was putting his nuclear forces on high alert, a senior U.S. defense official said on Monday.

But some former U.S. officials and experts caution that it would be a mistake to write off Putin's remarks as bluster, given the risk that Putin could decide to escalate to using nuclear weapons if he feels cornered over the war in Ukraine or if the war spills over into NATO.

Russia's defense ministry on Monday said its nuclear missile forces and Northern and Pacific fleets had been placed on "enhanced" combat duty, in line with an order the previous day from Putin.

The phrase special, or enhanced, combat duty appears to have stumped the Pentagon.

"It's not a term of art in what we understand to be Russian (nuclear) doctrine," the official said, speaking on condition of anonymity. "So that's why we're analyzing it and reviewing it to try to understand what exactly it means."

Leaders of the U.S. military, which built much of its intelligence collection architecture to spy on the Soviet Union, were not aware of Putin's decision until he made it publicly and, so far, there have not been big movements of weapons or forces to demonstrate what it means, the U.S. official said.

"I don't believe we've seen anything specific as a result of the direction that he gave, at least not yet, in terms of appreciable or noticeable muscle movements," the official said.

The United States closely monitors everything from Russian nuclear storage facilities to deployments of nuclear-capable bombers, missile forces and submarines.

The White House said it saw "no reason to change" its nuclear alert levels at this time, and President Joe Biden assured Americans that they should not fear nuclear war.

The United States and Russia account for more than 90% of the world's nuclear weapons, but only a fraction of them are deployed, according to the Union of Concerned Scientists.

Russia, which calls its actions in Ukraine a "special operation," has failed to achieve any obvious objectives five days after launching its invasion, with no cities under Russian control, no Russian dominance of airspace, and some Russian troops running out of fuel and supplies.

Putin is also confronting a wave of unprecedented economic and diplomatic isolation from the West, which is channeling hundreds of millions of dollars in weapons to Ukraine's military to fight Russian forces.


Jon Wolfsthal, a former top adviser to President Barack Obama on arms control, said the United States had long been concerned about Russia's nuclear weapons.

"We have to be very careful about what we do and don't do when you have one country that is backed into a corner, has nuclear weapons and is actually talking about their possible use," said Wolfsthal, currently with Global Zero, which advocates eliminating nuclear arms.

Michael McFaul, a former U.S. ambassador to Moscow and fierce Putin critic, wrote that it would be a mistake to dismiss Putin's message on nuclear arms.

"The people who know Putin the best -- people I know in Russia -- are worried about his recent nuclear statement. The people who know him the least are saying it's cheap talk," he wrote on Twitter.

A 2020 Russian revision to its nuclear doctrine suggested it might launch a nuclear response during a broader range of circumstances, including attacks on nuclear command and control, CRS, a nonpartisan research arm of Congress said in a report.

Robert Soofer, a former Pentagon nuclear policy adviser during the Trump administration, played down the risk that Russia would use nuclear weapons to address battlefield setbacks in Ukraine.

"The only way this is going to become a problem is if the war spills over into NATO," said Soofer, now at the Center for Strategic and International Studies think tank.

Francois Heisbourg, a senior adviser at the London-based International Institute for Strategic Studies (IISS), suggested Putin's threats should be taken seriously.

"People who say he is bluffing have only their gut feeling to rely upon. Whereas those who say he isn't bluffing can draw on a rich trove of circumstantial evidence," Heisbourg said.

"Because as far as Ukraine goes, he has not been bluffing. He doesn't do bluffing. He has been upfront in terms of what he wants."