A Russian convoy carrying food, water and other aid set off 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 into 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 said. "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 told journalists.
Andre Loersch, a spokesman for the International Committee of the Red Cross mission in Ukraine, said that while the organization had reached a general agreement about delivery of humanitarian aid to the region, he had "no information about the content" of the trucks and did not know where they were headed.
"At this stage we have no agreement on this, and it looks like the initiative of the Russian Federation," he said.
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 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 fervor in Russia through the state-controlled media since annexing Crimea in March, might be spurred to fresh military action since separatists in Donetsk are now encircled by Kiev government forces.
Rossiya 24 TV showed a nearly two-mile-long line of containers and trucks loaded with crates of water stretched along a road with workers loading sacks of aid. A Russian Orthodox priest marched across a line of trucks, spraying them with holy water before they left.
Itar-Tass news agency said the convoy was carrying 2,000 tons of humanitarian aid. It included 400 tons of cereals, 100 tons of sugar, 62 tons of baby food, 54 tons of medical equipment and medicine, 12,000 sleeping bags and 69 generators of various sizes.
"It has all been agreed with Ukraine," Business FM radio quoted Putin's spokesman, Dmitry Peskov, as saying of an operation publicized with fanfare on Russian TV channels.
Thousands of people are believed to be short of water, electricity and medical aid in Donetsk and in the border town of Luhansk due to bitter fighting involving airstrikes and missile attacks.
The United Nations says well over 1,000 people have been killed, including government forces, rebels and civilians, in the conflict in which a Malaysian airliner was downed on July 17, killing 298 people on board.
With Ukraine reporting Russia has massed 45,000 troops on its border, NATO said on Monday that there was a "high probability" Moscow might now intervene militarily in Ukraine.
French President François Hollande took the up issue directly with Putin, saying "he emphasized the strong fears evoked by a unilateral Russian mission in Ukrainian territory."
Hollande told Putin on Tuesday morning that any mission must be multilateral and have the agreement of the Red Cross and Ukraine, according to a statement.