A convoy of 262 Russian trucks rolled toward the Ukrainian border on Tuesday as Kiev vowed to block the aid mission from its territory over fears it was a ploy to bolster pro-Kremlin rebels.
The convoy of what Russia says is humanitarian aid has sparked fears of an escalation in a conflict that has left hundreds dead in eastern Ukraine in recent months and plunged Moscow's ties with the West to their lowest point in decades.
France warned Tuesday that Russia could use the operation as "a cover" for sending in troops, echoing earlier concerns from Washington, Berlin and London.
Kiev said the trucks would be stopped at the border for any aid to be unloaded and transported into conflict-torn eastern Ukraine with the help of the Red Cross.
"We will not allow (the aid) to be accompanied by the Russian ministry for emergency situations or by Russian troops," said Valeriy Chalyy, deputy head of the presidential administration.
However Moscow was adamant the convoy would reach its destination, calling for "maximum cooperation" from Ukraine to ensure the aid was delivered to the besieged rebel strongholds of Lugansk and Donetsk.
The three-kilometre-long (1.9-mile) aid convoy left Moscow on Tuesday for eastern Ukraine carrying 2,000 tonnes of "humanitarian supplies," including medical equipment, baby food and sleeping bags, Russian media reported.
Sources told Russian news agencies the convoy would arrive at the border on Wednesday.
As fierce fighting continued in the industrial east, Ukraine's military said six servicemen had been killed and 31 were wounded in the past 24 hours.
Seven civilians were also injured in shelling overnight in Donetsk, local authorities said, while Ukraine's military said it was ready to surround the rebels' second city of Lugansk.
Kiev's forces hope to cut off rebel access to the porous Russian border, where NATO says Moscow has massed 20,000 troops.
Fighting between government forces and pro-Moscow rebels also pushed Ukraine's currency, the hryvnia, to a historic low, trading at 13.14 against the dollar, although the central bank insisted this was only a "temporary trend".
- Confusion over aid mission -
Russia has been pressing for a humanitarian mission to the east, where four months of fierce battles have left cities without power, running water or fuel, and with dwindling food supplies.
President Vladimir Putin said Monday that Moscow was sending aid in response to the "catastrophic consequences" of Kiev's offensive against insurgents.
Kiev has said it will only accept aid as part of a broader international mission involving Europe and the US under the supervision of the Red Cross.
Moscow -- which denies allegations it is seeking to boost the insurgents who are losing ground to Ukraine's military -- has insisted it is working with the Red Cross and that the convoy would not include military personnel.
Russia has also said the route of the convoy had been agreed with Kiev.
"Russia expects maximum cooperation by the Ukrainian side in ensuring the security of the delivery of humanitarian aid," the foreign ministry said in a statement.
Adding to the confusion, the International Committee of the Red Cross said no green light had been given for an aid mission.
"We still need to get some more information before we can move ahead," ICRC spokeswoman Anastasia Isyuk told AFP in Geneva.
The Russian foreign ministry said the lorries would cross the border near the Russian town of Shebekino, into government-controlled territory, but there were fears the convoy could take a different route farther east into Ukraine, across a rebel-held stretch of the border.
- 'Grave concerns' -
French President Francois Hollande said he had "grave concerns" about the mission which his Foreign Minister Laurent Fabius warned "could be a cover by the Russians to set themselves up near Donetsk and Lugansk and declare a fait accompli."
The aid operation would be "only justifiable if the Red Cross gives its consent, if there are no military forces around (the mission), if there are not just Russianszbut other countries and if Ukraine agrees," Fabius added.  
"At this precise moment, this is not the case," he said.
Over 1,300 people have been killed in four months of what the Red Cross has already deemed a civil war, while 285,000 have fled their homes, according to the United Nations.