"Across the last two days we have seen the same thing that OSCE is reporting. We have seen columns of Russian equipment, primarily Russian tanks, Russian artillery, Russian air defence systems and Russian combat troops entering into Ukraine," US General Philip Breedlove said in Sofia.
"We do not have a good picture at this time of how many. We agree that there are multiple columns that we have seen," he added.
It was the latest in a string of recent sightings of unmarked trucks and heavy weapons heading towards the frontline in rebel-controlled areas.
Heavy artillery fire rocked Donetsk today. Ukraine's military said one soldier had been killed and two wounded, including one at the airport, as its positions came under repeated shelling around the region.
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"What worries me the most is that we have a situation now that the former international border between Ukraine and Russia is completely porous, it is completely wide open," Breedlove said.
"We need to get back to a situation where this international border is respected and that will help us to contain the problem of re-supply into eastern Ukraine.