German Chancellor Angela Merkel is due to arrive in Ukraine, a day after calling Russia's decision to send an unauthorised aid convoy there a "dangerous escalation".
More than 100 Russian lorries entered eastern Ukraine on Friday, without permission from Ukraine's government.
After travelling to besieged rebel-held areas, some of the lorries have now begun crossing back into Russia.
Western officials fear the convoy could be part of a military intervention.
But Kremlin officials say the vehicles are only carrying generators, food and drink.
The lorries had already been waiting at the border for a week, while Russia, the Ukrainian government and the Red Cross tried to come to an agreement on their passage.
The Russians said they could not wait any longer, owing to the worsening humanitarian situation in eastern Ukraine, which is held by pro-Russian separatists.
Four months of fighting in the region have left more than 2,000 people dead. More than 330,000 people have fled flee their homes.
At least 220 trucks drove into Ukraine on Friday, headed for the rebel-held city of Luhansk, which has been affected by weeks of fighting between Ukrainian forces and pro-Russian rebels.
International monitors based in the border region say that some of the vehicles have now crossed back into Russia, a development confirmed by Russian news agencies.
According to the Associated Press, journalists could see that about 40 of these returning lorries were empty - but it is unclear whether all of them are, and it also unclear what the convoy delivered.
The White House and the Ukrainian government both described the deployment of the convoy as a flagrant violation of Ukraine's sovereignty.
In a phone call, US President Barack Obama and Mrs Merkel said the conflict "has continued to deteriorate" since a Malaysian airliner was downed last month over rebel-held territory, with the loss of all 298 people on board.
Ukraine called the Russian convoy a "direct invasion" of Ukraine.
Nato and the European Union have also criticised what they said was a violation of Ukraine's sovereignty.
Nato officials have accused Russia of building up troops on its border, saying significant numbers of Russian forces are operating within Ukraine, using artillery.
But speaking to the UN Security Council, Russia's UN ambassador Vitaly Churkin accused Western powers of distorting reality.
"Sometimes it reminded me of the kingdom of crooked mirrors because some members of the Council were not concerned about the fact hundreds of civilians are dying."
He said Russia had to act to save perishable goods and that he hoped the Red Cross would help distribute the aid.
"We waited long enough. And it was time to move, and this is what we did," he said.