Ukraine on Friday accused Russia of invading its territory when a convoy of trucks Russia claims is carrying humanitarian aid crossed the border and headed for Luhansk, one of two cities controlled by separatists who seek Russian annexation.

Ukrainian president Petro Poroshenko described the entry of the trucks without his government’s consent as a “flagrant violation of international law.”

NATO called it a blatant breach of Russia’s international commitments and said the country had been using artillery against Ukrainian troops both across the border and within Ukraine, the closest the alliance has come to accusing its Cold War rival of war.

The risk of outright war between the two former Soviet states was highlighted last week when Ukraine said it partially destroyed an armored column that had crossed the border from Russia. However, Russia made no threat of retaliation and dismissed as “some kind of fantasy” the assertion that its troops had entered Ukraine.

Poroshenko said more than a hundred trucks crossed the border on Friday. Other officials said only 34 or 35 had been inspected.

Russia insisted the aid could simply not wait. Luhansk has been without electricity and water for more than two weeks.

A caravan of some 270 trucks was dispatched from Moscow ten days ago. Foreign journalists were able to look inside some of the vehicles when they were parked on the border with Ukraine in recent days and found several almost empty.

Part of the convoy was escorted by rebels who declared a breakaway republic in Luhansk which they hope Russia will annex. Russia occupied and annexed Ukraine’s Crimean Peninsula in March.

Last weekend, Ukrainian troops first entered Luhansk and shelled Donetsk, the only other city still held by the separatists.

Since Poroshenko assumed office in June, Ukraine’s forces have progressively driven back the rebels, prompting worries in Western capitals that Russia might intervene militarily to prevent the insurrection’s collapse.

Russia, which has thousands of troops close to its side of the border, warned against any attempt to “disrupt” what it said was a purely humanitarian effort on Friday.

The presence of the Russian trucks could force Ukrainian troops encircling Luhansk to suspend their offensive. If they hit one of the vehicles, that could possibly give Russia justification for an overt military operation.

If Russia only brings about a lull in the fighting, it could still give the separatists time to regroup.

The speed and success of the Ukrainian offensive in the east, which is home to the country’s ethnic Russian minority, appears to have taken the rebel leadership by surprise. Several Russian nationals who previously occupied key positions in the uprising have stood down.

Russia denies Western accusations that it backs the separatists but Russian artillery and tanks have found their way into Ukraine, as have missile launchers that were probably used by militants to shoot down a commercial airliner last month, killing nearly three hundred passengers and crew.