The White House has accused Russia of stepping up its destabilising activity in Ukraine in an "extremely dangerous and provocative" way.
But it stopped short of confirming Nato's assertion that a Russian military column had ventured into Ukraine overnight yesterday. The Ukrainian president said his forces destroyed most of the convoy.
National Security Council spokesman Caitlin Hayden said the US was not in position to confirm those reports but was working to gather more information.
Tensions have been rising in the region after Ukraine agreed to allow a Russian aid convoy to enter eastern Ukraine. 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.
The Pentagon said Russian defence minister Sergei Shoygu "guaranteed" to defence secretary Chuck Hagel yesterday that no Russian troops were involved in the transport of humanitarian relief supplies to eastern Ukraine.
In their first telephone conversation since late April, Mr Shoygu also assured Mr Hagel that the convoy of more than 250 trucks "was not to be used as a pretext to further intervene in Ukraine", said Pentagon spokesman, Rear Admiral John Kirby.
Rear Adm Kirby said Mr Shoygu "acknowledged that the goods would be delivered and distributed under the International Committee of the Red Cross", and Mr Shoygu assured Mr Hagel that "Russia was meeting Ukraine's conditions".
The convoy reports spooked global markets and overshadowed optimism driven by agreement over the Russian aid convoy bound for eastern Ukraine.
Throughout the eastern crisis that erupted in April, there have been consistent allegations that Russia is fomenting or directing the rebellion. Moscow denies this 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 speech on Thursday in which he said Russia hoped 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 armoured personnel carriers were seen crossing into Ukraine on Thursday night. Yesterday 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 said Russian military vehicles had entered Ukraine, but gave no specifics.
The Russian Defence Ministry insisted that no military vehicles were destroyed because none had crossed into Ukraine. Britain summoned Russian ambassador Alexander Yakovenko to clarify reports of the Russian incursion.
The crossing reportedly took place near the southern Russian town where the aid trucks had 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.
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.
But some Russian military vehicles near the aid convoy were seen yesterday 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 Mr Putin to end the flow of military goods and personnel into Ukraine ahead of a weekend meeting of foreign ministers aimed at easing tensions.
Ms Merkel's spokesman Steffen Seibert said the two leaders spoke before the meeting tomorrow between the Russian, Ukrainian, German and French foreign ministers in Berlin.
He said Ms Merkel urged Mr 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, authorities say. 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 at warehouses in the town of Starobilsk, 60 miles north of Luhansk, where the goods were to be sorted and transported further by the Red Cross.
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 in Lysychansk - some had windows blown out, while others had been blasted or burned to the ground.
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.