"The situation along the line of contact in Ukraine is not good. The fighting has intensified to essentially pre-agreement or pre-standdown levels and in some cases beyond," Breedlove said.
Following that accord, NATO said Russia had pulled back some of the troops it had inside Ukraine supporting pro-Russian separatists; but in recent days, Ukraine has said there has again been an increase of Russian forces in the country.
Breedlove said he could not confirm Ukrainian President Petro Poroshenko's estimate of 9,000 Russian troops in eastern Ukraine.
"What we do see is that the Russian-backed forces have renewed capability now to bring pressure on the Ukrainian forces and have in several places moved the line of contact to the west and this is concerning," he said.
"We are beginning to see the (heat) signatures of air defense systems and electronic warfare systems that have accompanied past Russian troop movements into Ukraine," he said.
Russia denies deploying troops in eastern Ukraine, where more than 4,800 people have been killed since a pro-Russian rebellion broke out soon after Moscow annexed Crimea in March. It also rejects accusations it is supplying arms.