Hagel: Russian invasion of Ukraine a possibility

Defense Secretary Chuck HagelCharles (Chuck) Timothy HagelPentagon documents hundreds of serious misconduct cases against top brass Obama defense sec: Trump's treatment of Gold Star families 'sickens' me The Hill's 12:30 Report MORE on Wednesday said Russia’s troop buildup along Ukraine’s eastern border increases the chances of Moscow launching a ground invasion.

"I think it's a reality, of course it is," Hagel said while visiting U.S. European Command in Stuttgart, Germany, The Associated Press reported.

“When you see the buildup of Russian troops and the sophistication of those troops, the training of those troops, the heavy military equipment that's being put along that border, of course it's a reality, it's a threat, it's a possibility — absolutely," he said.

Hagel is the most senior U.S. official to sound the alarm about roughly 20,000 Russian troops positioned just along the border with Ukraine. Earlier Wednesday, a NATO spokeswoman said the alliance was concerned the Kremlin could order an invasion under the auspices of a peace-keeping mission.

Russia has repeatedly denied accusations that it is massing soldiers at the border or providing lethal weapons to pro-Russian separatists in eastern Ukraine.

Hagel visited Germany to meet with U.S. military chiefs in Europe, including Air Force Gen. Philip Breedlove, NATO’s supreme allied commander, as part of a nine-day trip that will also take him to India and Australia.

"The longer that Russia perpetuates and instigates this tension and the possibility of escalating their activity, it's going to get worse. And we have to be prepared for that," Hagel said.

Russia’s latest move has forced the U.S. and its allies to examine how NATO is structured to respond to such military challenges, he added.