NATO on Friday said a Russian military column ventured overnight into Ukraine, and the Ukrainian president said his forces destroyed most of it. Russia denied all of this, but the reports spooked global markets and overshadowed optimism driven by agreement over a Russian aid convoy bound for eastern Ukraine.

The White House said it was looking into what it called unconfirmed reports that Ukraine's security forces disabled vehicles in a Russian military convoy inside Ukraine.

The Russian aid convoy of more than 250 trucks has been a source of tensions since it set off from Moscow on Tuesday. Kiev and the West were suspicious that the mission could be a pretext for a Russian military incursion into eastern Ukraine, where government forces are battling pro-Russia separatists and clawing back rebel-held territory.

Throughout the eastern crisis that erupted in April, there have been consistent allegations that Russia is fomenting or directing the rebellion. Moscow rejects the allegations and the high-profile aid convoy could be aimed, in part, at portraying Russia as interested in cooling the conflict. Russian President Vladimir Putin appeared to cultivate that perception in a Thursday speech in which he said Russia hopes for peace in Ukraine.

It was not clear what Russia could hope to gain by sending in a military column while world attention was trained on its efforts to get the aid convoy into eastern Ukraine.

But some foreign journalists reported that Russian armored personnel carriers were seen crossing into Ukraine on Thursday night. On Friday, a statement on Ukrainian President Petro Poroshenko's website said "the given information was trustworthy and confirmed because the majority of the vehicles were destroyed by Ukrainian artillery at night."

NATO secretary-general Anders Fogh Rasmussen also confirmed that Russian military vehicles had entered Ukraine, but he gave no specifics.

In Moscow, a spokesman for the Russian Defense Ministry insisted that no Russian military vehicles were destroyed because none had crossed into Ukraine. Yet Britain said it summoned Russian Ambassador Alexander Yakovenko in to clarify reports of the Russian incursion.

In Washington, National Security Council spokeswoman Caitlin Hayden said the U.S. government is working to gather more information about the reports. She said the U.S. remains concerned about repeated Russian and Russian-supported incursions into Ukraine.

Markets sold off heavily Friday, spooked by thought of Ukrainian troops engaging with Russia forces inside Ukraine. Germany's DAX, which had been trading over 1 per cent higher, ended the day 1.4 per cent lower.

The crossing reportedly took place near the southern Russian town where the aid trucks have been parked, awaiting permission to go into Ukraine.

After days of controversy, Russia nominally consented to let Ukrainian officials inspect the convoy while it was still on Russian soil and agreed that the Red Cross would distribute the goods in Ukraine's region of Luhansk.

In Washington, a Pentagon spokesman said Russian Defense Minister Sergei Shoygu "guaranteed" Defense Secretary Chuck Hagel on Friday that no Russian troops are involved in the transport of humanitarian relief supplies to eastern Ukraine.

Ukraine proceeds with own aid mission

In their first telephone conversation since late April, Shoygu assured Hagel that the Russian convoy "was not to be used as a pretext to further intervene in Ukraine," according to the spokesman, Navy Rear Adm. John Kirby. Kirby did not mention whether the two discussed Ukraine's claim that it had attacked Russian military convoy vehicles.

Laurent Corbaz, the International Committee of the Red Cross' director of operations in Europe, described a tentative plan in which the trucks would enter Ukraine with a single Russian driver each — as opposed to the current crew of several people in each truck — accompanied by a Red Cross worker. In line with Red Cross policy, there would be no military escort, he said.

However, some Russian military vehicles near the aid convoy were seen Friday carrying a Russian acronym standing for "peacekeeping forces" — a signal that Moscow was considering a possible military escort.

Germany, meanwhile, said Chancellor Angela Merkel pressed Putin to end the flow of military goods and personnel into Ukraine ahead of a weekend meeting of foreign ministers aimed at easing tensions.

Merkel spokesman Steffen Seibert said the two leaders spoke Friday evening ahead of a meeting Sunday between the Russian, Ukrainian, German and French foreign ministers in Berlin.

He said Merkel urged Putin to de-escalate the situation "and in particular put an end to the flow of military goods, military advisers and armed personnel over the border into Ukraine."

The fighting in eastern Ukraine has claimed nearly 2,100 lives, half of those in the last few weeks as the Ukrainian troops regained more and more rebel-held territory. It began in April, a month after Russia annexed Ukraine's Black Sea peninsula of Crimea.

The eastern Ukrainian city of Luhansk has suffered extensively from an intense military barrage over the last few weeks. The city remains cut off from power and water supplies, and its mobile and landline telephone systems barely function, local authorities said Friday. Little food is available but bread is still being made using portable generators.

Ukraine, meanwhile, proceeded with its own aid mission to the Luhansk area. Trucks sent from the eastern city of Kharkiv were unloaded Friday at warehouses in the town of Starobilsk, where the goods were to be sorted and transported further by the Red Cross. Starobilsk is 100 kilometres north of Luhansk.

Other Ukrainian aid was taken to the town of Lysychansk, which retaken by Ukrainian forces late last month but has seen sporadic clashes until earlier this week.

Dozens of houses showed signs of damage Friday in Lysychansk — some had windows blown out, while others had been blasted or burned to the ground. An Associated Press reporter saw small children playing in the rubble of one destroyed house.

As Ukrainian emergency workers discussed how to distribute the aid, clusters of older women and small children began appearing on the town's streets. Residents said the aid was the first they had seen since fighting had ended.