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 Kyiv said it would not allow the vehicles to cross onto its territory.
Kyiv 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.
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 action since separatists in their main redoubt of Donetsk are now encircled by Kyiv government forces.
Rossiya 24 TV showed an approximately 2-mile long line of containers and trucks loaded with crates of water stretched along a road with workers in a light summer wear of shorts and shirts 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 publicized with fanfare on Russian TV channels.
The US, 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 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.
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.
UN 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.
Kyiv and its Western allies say Russia, which opposes the new leadership's pro-Western policies, has been funneling tanks, missiles and other heavy weapons to the rebels for months. Moscow denies this.
Suspicious of Russian intentions, Ukrainian President Petro Poroshenko has said Ukraine will allow Russian aid into the country only as part of an international relief mission, under the auspices of the International Committee of Red Cross (ICRC) and involving the United States, European Union and other states.
In Geneva, an ICRC spokesperson said: "We still need clarification, details on the content (of the convoy) and type of transportation and storage needed."
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.
A former Ukrainian president, Leonid Kuchma, who has been involved in tentative mediation with the rebels said on Tuesday the convoy would travel down through north-eastern Ukraine to Luhansk under an Organization for Security and Cooperation in Europe (OSCE) escort.
Speaking in Sydney on Tuesday, US Secretary of State John Kerry said: "Our hope is that in the next days and weeks, we can find a way for President 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: "Wemust 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."
"This (convoy) is only possible, only justifiable, if the Red Cross authorizes it," he told France Info Radio.
US President Barack Obama said on Monday that any Russian intervention without Kyiv's consent would be unacceptable and violate international law.
European Commission President Jose Manuel Barroso also warned on Monday "against any unilateral military actions in Ukraine, under any pretext, including humanitarian."
(Additional reporting by Natalia Zinets in Kyiv, Stephanie Ulmer-Nebehay in Geneva and Polina Devitt in Moscow; Writing by Richard Balmforth)