'I don't believe it's a bluff,' Defense Secretary Austin says of a possible Russian invasion of Ukraine

"This is not a bluff," Defense Secretary Lloyd Austin said Friday about the potential for a Russian invasion of Ukraine in an exclusive interview with ABC "This Week" co-anchor Martha Raddatz.

"He has a number of options available to him and he could attack in short order," Austin told Raddatz in an interview conducted in Powidz, Poland, where Austin had come to visit American and Polish troops.

Raddatz's full interview with Austin airs Sunday on a special edition of "This Week" from Lviv, Ukraine.

"This is not a bluff?" Raddatz asked Austin about the potential for a Russian invasion.

"I don't believe it's a bluff," Austin replied.

"I think it's ... I think he's assembled the right kind -- the kinds of things that you would need to conduct a successful invasion," he added.

Tensions over the potential for a Russian invasion in Ukraine have increased as American officials have repeatedly said that a Russian invasion could occur "any day."

Russia's defense ministry claimed this week that it had begun withdrawing tank units and other forces from the area at the conclusion of exercises that had been cited as the reason for the massive troop buildup around Ukraine.

But U.S. officials have rejected those claims, saying instead that Russia has continued to flow more troops into the border area while it was claiming a troop withdrawal was taking place.

"He's got things like medical tents and nurses -- you ... would never have seen that if you were going to be withdrawing your troops?" Raddatz asked Austin.

"Exactly. If they were redeploying to garrison, we wouldn't be seeing the kinds of things in terms of, not only combat power, but also logistical support, medical support, combat aviation that we've seen in the region," Austin responded.

On Friday, the American representative to the Organization for Security Cooperation in Europe reported to that organization that the U.S. assesses that Russia has "probably" amassed between a force numbering between 169,000 and 190,000 near Ukraine's borders, a total that includes Russian separatist forces in eastern Ukraine.

The number of Russian separatists forces has not previously been included in estimates of Russian troop levels provided by American officials.