Russia has "guaranteed" that no military personnel are on a convoy waiting at the border with Ukraine to deliver aid to the conflict-hit east of the country.
Ukraine has been concerned the convoy of about 260 white trucks could be a 'Trojan horse', which will allow Russia to set up a permanent presence in rebel-held territory.
Officials from the Red Cross have been examining the contents of the trucks on the Russian side of the border to make sure they contain nothing other than aid.
US Defence Secretary Chuck Hagel said he spoke with his Russia counterpart on Friday night and requested clarification on the convoy.
Russian truck drivers check their vehicles, which are said to contain aid A statement released by the Pentagon, about the phone call between Mr Hagel and Sergey Shoygo, said: "Minister Shoygu 'guaranteed' that there were no Russian military personnel involved in the humanitarian convoy, nor was the convoy to be used as a pretext to further intervene in Ukraine.
"He acknowledged that the goods would be delivered and distributed under the International Committee of the Red Cross.
"Minister Shoygu assured Secretary Hagel that Russia was meeting Ukraine's conditions."
Russian officials allowed journalists to see the contents of some trucks It came after Russia angrily denied it had sent a separate armed convoy into Ukrainian territory that Ukraine claims it partially destroyed.
Ukrainian President Petro Poroshenko said his forces had destroyed part of the convoy adding: "We won't tolerate any invasion."
Officials in Kiev said they tracked the vehicles, including armoured personnel carriers, from the border and then attacked with artillery.
Ukrainian guards check people suspected of crossing the border illegally The claim was partially verfied by journalists from the Guardian and Daily Telegraph newspapers who said they saw around 23 Russian military vehicles crossing the border near the town of Donetsk on Thursday night.
Sky's Foreign Affairs Editor Sam Kiley said any attack would mark "a very serious development" in the four-month conflict, and "could be the beginnings of something much more dangerous".
The US later said it was not able to confirm whether Kiev's forces had attacked the convoy, but said Russia had no right to send vehicles into Ukraine.
White House spokesperson Caitlin Hayden said: "Russia has no right to send vehicles, persons, or cargo of any kind into Ukraine, under any pretext, without the Government of Ukraine's permission."

Moscow's Defence Ministry dismissed Kiev's claim that it had sent a convoy into Ukraine as "some kind of fantasy".
Spokesman Igor Konashenkov told Ria Novosti agency: "There was no Russian military column, which allegedly crossed Russian-Ukrainian border, not in the night, not during the day, it just doesn't exist."
The European Union said it would consider any Russian incursion as "a blatant violation of international law".
Fighting continued on Friday with 11 civilians killed and eight more wounded by shelling in the besieged stronghold of Donetsk in 24 hours.
Ukraine's Foreign Minister Pavlo Klimkin is due to meet his Russian counterpart Sergei Lavrov in Berlin on Sunday to discuss the crisis.
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