U.S. Gen. Philip Breedlove said a large number of Russian troops are also active inside Ukraine, training and advising separatist rebels.
Moscow has routinely denied it played any direct role in the conflict in Ukraine, which has claimed more than 4,000 lives.
Breedlove spoke during a brief visit to Kiev, where he met with top officials to discuss continued NATO assistance for Ukraine in its fight against Russian-backed separatists in the east.
"We are going to help Ukraine's military to increase its capacities and capabilities through interaction with U.S. and European command," he said, adding that it "will make them ever more interoperable with our forces."
Tony Blinken, deputy U.S. national security adviser, said he believed Washington ought to consider strengthening Ukrainian forces as a message to Moscow. Providing defensive military equipment to Ukraine has broad support in the U.S. Congress.
The Ukrainian armed advance against rebel forces ground to a halt and was substantially reversed over the summer as government troops found themselves faced by a well-equipped and determined foe.
"This international border is completely wide open and maintained open by Russian forces, so that forces, supplies, money, fighters can move across at will," Breedlove said.
A truce was agreed in early September but fighting continues daily in several areas of eastern Ukraine.