"Ukraine has a vastly more capable and larger force than does the country that is more than three times their size," Petraeus told CNBC.

"The Ukrainians have very, very good snipers and they’ve just been picking them off left and right,” retired U.S. Gen. David Petraeus said on CNN.

The former commander of U.S. forces in Afghanistan appeared to quash rumors that he will succeed Jim Mattis as secretary of defense.