Social video of Russian military vehicles gathered around the Ukrainian border, while trucks carrying humanitarian aid headed for the Ukrainian cities of Luhansk and Donetsk were inspected by border patrol. (Facebook/Catherine Makarevich)

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 defense ministry spokesman said that no Russian vehicles had been destroyed because none had been inside Ukraine. Poroshenko offered no proof to support his claim.

In Washington, the White House said late Friday that it could not confirm reports of a Ukrainian attack on a Russian convoy in its territory and was “working to gather more information.”

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 a 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.

“Those who harbor such criminal plans are taking on a huge responsibility for the consequences,” the statement said.

On Friday, Defense Secretary Chuck Hagel called Sergei Shoigu, the Russian defense minister, to ask about the humanitarian convoy headed toward Ukraine. According to a statement from the Pentagon, Shoigu “guaranteed” Hagel that there were no Russian military personnel involved with the convoy and that the aid trucks were not being used as a pretext to intervene militarily in Ukraine.

But the convoy appears to face numerous bureaucratic hurdles before it can actually reach its destination. The International Committee of the Red Cross (ICRC) is supposed to distribute the aid, but Russian authorities still have not provided a detailed description of what had been sent, ICRC spokesman Andre Loersch said Friday.

That makes it impossible for the group to plan the logistics of distributing it, he said. Nor did his group have security guarantees from the rebels, a necessary step before entering a zone where there is intense fighting.

“It will take time, and it’s a big challenge,” Loersch said, adding that it could be weeks before the aid actually makes it to Luhansk. Red Cross officials have said that the Russian trucks must deposit their cargoes in a central location and then immediately return to Russia via the same route.

Heavy fighting continued Friday inside the rebel stronghold city of Donetsk and near the second stronghold of Luhansk, witnesses and Lysenko, the military spokesman, said. Donetsk city authorities said Friday that 11 civilians were killed in heavy shelling over the previous day. Lysenko said that five Ukrainian troops were also killed.

With the Russian vehicles poised to enter Luhansk, it seemed clear that the Ukrainian forces would be unable to continue combat operations in the area at a pace similar to their tempo in recent days. The aid trucks are painted white but have Russian military license plates; if one were hit by Ukrainian forces, it could set off a Russian invasion.

Lysenko declined to comment on how the convoy would affect combat on the ground, though he said that final preparations have been made to retake both cities. He said that decisions about how the aid is distributed would be up to the Red Cross.

The foreign ministers of Ukraine, Russia, Germany and France planned to meet Sunday in Berlin to discuss the situation in Ukraine.

Karoun Demirjian in Moscow and Craig Whitlock and Karen DeYoung in Washington contributed to this report.