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.
Rossiya 24 TV showed a 3-km long line of containers and trucks loaded with crates of drinking water and other products 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.
"It has all been agreed with Ukraine," Business FM radio quoted Putin's spokesman, Dmitry Peskov, as saying of an operation publicised with fanfare on Russian TV channels.
The U.S., French and Australian governments voiced concern that Russia, sole international supporter of rebels in Ukraine's Russian-speaking east, could use the humanitarian deliveries to carry out a covert operation to help fighters who appear to be on the verge of defeat.
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.
Itar-Tass news agency said the convoy carried 2,000 tonnes of humanitarian aid. It included 400 tonnes of cereals, 100 tonnes of sugar, 62 tonnes of baby food, 54 tonnes of medical equipment and medicine, 12,000 sleeping bags and 69 generators.
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 air strikes and missile attacks.
U.N. agencies say 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 with the deaths of all 298 people on board.
A further six Ukrainian soldiers were killed in action overnight, military spokesman Andriy Lysenko said, bringing to more than 570 the number of government forces killed since conflict erupted in April.
Kiev and its Western allies say Russia, which opposes the new leadership's pro-Western policies, has been funnelling tanks, missiles and other heavy weapons to the rebels for months. Moscow denies this.
With the humanitarian crisis growing by the day in eastern Ukraine, the Kremlin might welcome the opportunity to garner sympathy if the Russian convoy arrives at the border and is turned away by the Ukrainians.
If, and when, the cargo crosses into Ukraine it is expected to come through a border crossing point just north of the city of Kharkiv.
A former Ukrainian president, Leonid Kuchma, who has been involved in tentative mediation with the rebels said it would then travel down through north-eastern Ukraine to Luhansk under an Organization for Security and Cooperation in Europe (OSCE) escort.
Speaking in Sydney on Tuesday, U.S. Secretary of State John Kerry said: "Our hope is that in the next days and weeks, we can find a way for President Petro Poroshenko and Ukraine to be able to work with the Russians to provide the humanitarian assistance necessary in the east."
Australian Foreign Minister Julie Bishop told reporters: "Any intervention by Russia into Ukraine under the guise of an humanitarian crisis would be seen for the transparent artifice that it is and Australia would condemn (it) in the strongest possible terms."
In Paris, French Foreign Minister Laurent Fabius said: "We must be extremely careful because this (the convoy) could be a cover for the Russians to install themselves near Lugansk and Donetsk and put us before a done deed."