A Russian convoy of trucks carrying tonnes of humanitarian aid left on Tuesday for eastern Ukraine, where government forces are closing in on pro-Russian rebels, but Kiev said it would not allow the vehicles to cross onto its territory.
Kiev and Western governments warned Moscow against any attempt to turn the operation into a military intervention by stealth in a region facing a humanitarian crisis after four months of warfare.
"This cargo will be reloaded onto other transport vehicles (at the border) by the Red Cross," Ukrainian presidential aide Valery Chaly told journalists.
"We will not allow any escort by the emergencies ministry of Russia or by the military (onto Ukrainian territory). Everything will be under the control of the Ukrainian side," he said.
Russia said it would transfer the convoy to the aegis of the International Committee of the Red Cross, but made no reference to the demand the goods be reloaded. The European Union said the aid would have to be verified.
"No political or any other objectives must be pursued," EU humanitarian aid commissioner Kristalina Georgieva told a news conference. "The content of humanitarian aid must be exactly that, humanitarian aid, and obviously cannot be taken on face value."
Russian media said the column of 280 trucks had left from near Moscow and it would take a couple of days for it to make the 1,000 km (620 mile) journey to Ukraine's eastern regions where rebel fighters seek union with Russia.
Western countries believe that Russian President Vladimir Putin, who has whipped up nationalist fervour in Russia through the state-controlled media since annexing Crimea in March, might be spurred to fresh action since separatists in their main redoubt of Donetsk are now encircled by Kiev government forces.