KIEV, Ukraine — Ukrainian President Petro Poroshenko said Friday that Ukrainian forces had attacked and destroyed part of a column of Russian military vehicles on Ukrainian territory, a step that, if confirmed, would represent a significant escalation of hostilities between Ukraine and Russia.

Poroshenko told British Prime Minister David Cameron that “the majority” of a column of Russian military vehicles “had been destroyed by the Ukrainian artillery at night,” his office said in a statement. The announcement came as NATO Secretary-General Anders Fogh Rasmussen said Friday that the defense alliance had seen an “incursion” into Ukraine the previous night.

The Kremlin denied Friday that there had been a border incursion, and a Russian military spokesman said that no Russian vehicles had been destroyed.

Kiev has claimed for months that Russia has been sending a stream of heavy weaponry to pro-Russian rebels who have seized territory in eastern Ukraine, but the Thursday encounter appeared to be the first time Western reporters had actually witnessed a military incursion. And unlike other rebel-held military vehicles, which have been stripped of markings, the armored personnel carriers seen crossing the border bore full Russian identification, including license plates, according to journalists with Britain’s Guardian and Telegraph newspapers who said they saw the vehicles cross the border.

Poroshenko said that Ukrainian forces had engaged the same Russian column of vehicles that the journalists had witnessed entering Ukraine. It was not immediately clear how he knew that it was the same cluster of vehicles, and neither his account nor that of the British journalists could be independently confirmed.

The apparent head-to-head military engagement would mean that Ukrainian and Russian forces had directly engaged in combat on Ukrainian soil.

The combat came as Ukrainian border guards prepared Friday to inspect a vast convoy of Russian humanitarian aid bound for war-torn eastern Ukraine.

The compromise deal to allow the Ukrainian officials to inspect the vehicles on Russian soil initially will enable the trucks to cross straight into rebel-held territory near the besieged city of Luhansk. The convoy of 262 Russian vehicles had driven close to a rebel-held border checkpoint a day earlier.

But the humanitarian aid also appeared to be a source of tension on Friday, as the Russian Foreign Ministry warned that it had received reports that Ukrainian forces planned to target its aid trucks once they entered Ukrainian territory.

“There are forces that are not simply bent on depriving the population of southeastern Ukraine of much-needed aid now, but also on carrying out an open provocation,” the Foreign Ministry said in a statement. It said it had “received reports of direct threats to use force against our convoys,” saying that a pro-Kiev volunteer battalion planned to mine stretches of the road that runs between the Russian border and the rebel-held city of Luhansk, the aid convoy’s final destination.

Rebels control most of the territory between the border crossing and Luhansk, but the Ukrainian military said Thursday that it had retaken a key town abutting the main transit highway in the area. The Ukrainian military said Friday that a team of 59 border and customs officials had traveled to the Russian side of the border crossing to prepare for screening the trucks.

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