The conflict between Russia and Ukraine hit dangerous new heights Friday as Russia sent an enormous aid convoy into rebel-held Ukrainian territory without the permission of the Kiev government, a move that a top Ukrainian security official described as a “direct invasion.” The maneuver came amid reports from NATO that Ukrainian troops were coming under Russian artillery fire from inside their borders.

“Russian artillery support — both cross-border and from within Ukraine — is being employed against the Ukrainian armed forces,” NATO Secretary General Anders Fogh Rasmussen said in a statement accusing Moscow of a “blatant breach of Russia’s international commitments” that would intensify a crisis he said it had helped to create and fuel.

“The disregard of international humanitarian principles raises further questions about whether the true purpose of the aid convoy is to support civilians or to resupply armed separatists,” Rasmussen said. It was the strongest denunciation of Russia’s role in Ukraine that NATO has issued and the first time the alliance has accused Russian forces of firing artillery at the Ukrainian army from within Ukraine.

The charges coincide with Russia’s decision to move a convoy of more than 200 trucks into Ukraine on Friday without either government permission or the participation of the International Committee of the Red Cross (ICRC).

Russia’s Foreign Ministry said in a statement Friday morning that Moscow had run out of patience with “delays” and other “excuses” from Ukraine after a nearly 10-day standoff. It said Ukraine’s leaders were deliberately trying to slow-walk the delivery of aid to the war-torn region of Luhansk until “there is no one at all to provide help to.”

The White House condemned the Russian action and said it raised the likelihood that Russia planned the convoy as a pretext for invasion.

“At the same time as Russian vehicles violate Ukraine’s sovereignty, Russia maintains a sizeable military force on the Ukrainian border capable of invading Ukraine on very short notice,” National Security Council spokeswoman Caitlin Hayden said. “We recall that Russia denied its military was occupying Crimea until it later admitted its military role and attempt to annex this part of Ukraine.”

U.S. Air Force Gen. Philip M. Breedlove, the supreme allied commander in Europe, separately condemned “Russia’s illegal incursion” into Ukraine as he expressed concern about the massing of 20,000 Russian “combat-ready troops” on the border with eastern Ukraine and the flow of Russian arms and operatives to pro-Moscow separatist forces. The unauthorized convoy “indicates that Russia is more interested in resupplying separatists rather than supporting local populations,” Breedlove said in a statement.

Ukrainian authorities appeared to be scrambling Friday to decide how to respond to the border incursion.

Officials had threatened a military response if the Russian convoy tried to force its way into Ukraine, despite the risk of triggering an all-out invasion by Russian forces. Yet allowing the trucks to disperse across the Luhansk region without any Ukrainian controls in effect allows Russia to force a cease-fire in Kiev’s fight against pro-Russian separatists.

State security chief Valentyn Nalyvaichenko told journalists in Kiev that Ukrainian forces would not use force against the convoy because they want to avoid “provocations.” But Ukraine’s prime minister struck a more confrontational tone.

“It’s clear that Russia is not planning to conduct any humanitarian mission,” Arseniy Yatsenyuk said on national television. “We need to use all methods to stop Russian military aggression.”

For Ukrainian officials, a potential military response ultimately depends on whether the Russian convoy tries to do anything more than distribute aid.

“If we find in the convoy some other equipment, some other equipment that’s not humanitarian aid, then the direction will be different,” military spokesman Andriy Lysenko said.

On Friday evening, Russian state television broadcast images of the trucks pulling up to a loading dock in rebel-held Luhansk. Shirtless men wearing jeans unloaded large white bags that appeared to be filled with grain, beans or flour and moved them into a warehouse.

The original ICRC plan was for the trucks to enter Ukraine, deposit aid supplies and leave immediately by the same route. Russia’s ambassador to the United Nations, Vitaly Churkin, declined to say whether the convoy would hold to that plan.

Meanwhile, Ukraine’s Foreign Ministry urged international allies to condemn Russia for what Ukrainian President Petro Poroshenko called a “flagrant violation of international law.”

In Washington, a Pentagon spokesman, Rear Adm. John Kirby, stopped short of calling the movement an invasion but said “it strains credulity to think that this equipment’s not moving across the border accompanied by Russian forces.”

Kirby called on Russia to withdraw vehicles and personnel and threatened “additional costs and isolation” otherwise. That is a reference to potential further economic sanctions on Russia and diplomatic ostracizing of Moscow, tactics the West has applied for months with little success.

“They should not be doing this under the guise of a humanitarian convoy,” Kirby said.

In a conversation with German Chancellor Angela Merkel, Russian President Vladimir Putin said “explicit delays from the side of Kiev” forced Russia to send the convoy across the border unilaterally. Further delays in getting help to Luhansk residents, many of whom have no water or electricity, would be “unacceptable,” he added.

The latest delays, however, emanated from the ICRC’s concerns for the safety of its workers.

Poroshenko had agreed last week to let Russian and European aid into rebel-held portions of the eastern region of Luhansk, but only if ICRC workers presided over the shipments. The ICRC asked for security guarantees, which Ukraine gave — but only for areas under government control.

On Friday morning, ICRC officials told Russia that after a night of heavy shelling in Luhansk, they did not yet have the necessary safety guarantees.

Yet the convoy was shrouded in controversy from the start. When Russia first sent the trucks toward Ukraine they did it without specific ICRC authorization, prompting Ukraine and its allies to worry that the shipment was designed as cover for a military invasion.

Ukrainian officials refused to let the trucks through a government-controlled border crossing in the Kharkiv region, so the convoy headed for rebel-controlled territory instead.

The trucks then spent more than a week idling outside the Izvaryne border crossing to Luhansk, which is controlled by pro-Russian rebels, as government ministers tried to hammer out a deal.

Reporters in the area noted the presence of food, water and emergency supplies in the trucks. Some on the scene also reported seeing heavy military vehicles near the convoy on the Russian side of the border.

Last week, Western reporters watched a column of Russian armored personnel carriers enter Ukraine while the aid convoy was waiting on the Russian side. Ukraine said its forces subsequently destroyed part of the column, but Russia denied that any of its vehicles had crossed the border or had been attacked.

On Thursday, Russian customs officials cleared the first group of aid trucks into the border zone between Russian and Ukrainian territory, where they were waiting for the go-ahead to enter Ukraine.

That night, ICRC officials said the first trucks would likely enter Ukraine on Friday.

William Branigin, Anne Gearan and Karen DeYoung in Washington and Stephanie Kirchner in Berlin contributed to this report.