Russia's defence secretary has assured his US counterpart that there are no military personnel in its controversial aid convoy for Ukraine, the US says.
It said Sergey Shoygu told Chuck Hagel the convoy was not being used as a pretext to intervene further .
The convoy, which aims to aid eastern Ukrainian cities held by pro-Russian rebels, is still stalled at the border.
Earlier Russia denied Ukraine's claims that a column of Russian armoured vehicles had crossed the border.
'Will take some time'
The Pentagon said Mr Hagel had sought clarification on the convoy.
It 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."
It said: "Minister Shoygu assured Secretary Hagel that Russia was meeting Ukraine's conditions."
Ukraine insists on a full inspection. Its border guards have arrived at the convoy but have not yet cleared it to pass.
The BBC's Steve Rosenberg, who has been following the convoy, says some of the lorries were opened up for the media on Friday, and the main thing that struck him was how empty many of the vehicles were.
The ICRC's Head of Operations for Europe and Central Asia, Laurent Corbaz, said: "The agreement... foresees that the trucks will be checked by the customs officers of Ukraine on Russian territory and then be allowed inside Ukraine with ICRC people accompanying the convoy and then unload somewhere in Luhansk and go back to Russia, empty."
He said that given the size of the cargo and the security constraints, "implementation is likely to take some time", not in one week and possibly "much longer".
Russia complained on Friday about Ukrainian operations in the area.
The Russian foreign ministry said: "We draw attention to the sharp intensification of military action by Ukrainian forces with the apparent aim to stop the path, agreed on with Kiev, of a humanitarian convoy across the Russia-Ukraine border."
Ukraine in turn said it had partially destroyed an armoured column that had crossed from Russia overnight on Thursday.
The alleged incursion was witnessed by two UK newspaper reporters.
Russia's defence ministry said the incursion reports were "some kind of fantasy".
It said: "There was no Russian military column that crossed the Russian-Ukrainian border either at night or during the day."
The incident prompted sharp words from Nato and the UK.
Nato Secretary General Anders Fogh Rasmussen said: "We see a continuous flow of weapons and fighters from Russia into eastern Ukraine, and it is a clear demonstration of continued Russian involvement in [its] destabilisation."
The UK Foreign Office summoned Russian Ambassador Alexander Yakovenko to explain.
EU foreign ministers meeting in Brussels issued a statement saying: "Any unilateral military actions on the part of the Russian Federation in Ukraine under any pretext, including humanitarian, will be considered by the European Union as a blatant violation of international law."
Russia's government has consistently denied directly arming or training the rebels.
However German Chancellor Angela Merkel told Russian President Vladimir Putin in a phone call on Friday to "put an end to the flow of military goods, military advisers and armed personnel over the border".
The conflict in Ukraine's east, which has claimed more than 2,000 lives, has intensified in recent weeks.
The violence began in April when pro-Russian rebels seized government buildings and tried to declare independence.
The Ukrainian military launched an operation to retake the region and stepped up its activities in June.