Austin was speaking at a press conference in Seoul where he was asked about Russia massing troops along its boundary with Ukraine, with fears swirling of a possible invasion.

"Russia has a substantial amount of forces in the border region, and we remain concerned about that," Pentagon chief Lloyd Austin said.

"The best case though is that we won't see an incursion by the Soviet Union into Ukraine," he added, accidentally using the name of the country that disintegrated in 1991 into republics including Russia and Ukraine.

"And so we would hope that (Russian President Vladimir) Putin would be a lot more transparent and that we would work to resolve issues and concerns, and lower the temperature in the region."

The Cold War between the United States and the Soviet bloc was the defining conflict of the post-World War II era -- until the latter collapsed.

It saw a huge build-up of arms -- including atomic weapons -- on both sides as well as proxy conflicts, with fear of nuclear war swirling for decades.

The end of the Soviet Union brought a new era in Washington-Moscow relations, but the rise of Putin this century has coincided with tensions not seen since the Cold War.

The top American and Russian diplomats are to meet in Sweden on Thursday, with US Secretary of State Antony Blinken saying ahead of the talks that he was "deeply concerned" by the Russian build-up.

Moscow, accused of backing the separatists fighting Kiev, has denied preparing an attack and accused NATO of stoking tensions.