MOSCOW - A convoy of 262 Russian trucks was expected at the Ukrainian border on Wednesday 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.
The United States backed Ukraine's demand for border checks to clear up concerns that Moscow is seeking to support rebels.
Washington and its European allies have been voicing concern for days over the convoy, which sources told Russian news agencies would arrive at the border on Wednesday.
"Russia has no right to move into Ukraine unilaterally, whether under the guise of humanitarian convoys or any other pretext, without Kiev's permission," State Department spokeswoman Marie Harf told reporters.
Ukrainian officials "have a plan in place that they feel comfortable with. We feel comfortable with it as well. And now the Russians need to deliver," Harf said.
France warned earlier that Russia could use the operation as "a cover" for sending in troops, echoing concerns from 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 International Committee 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 carrying 2,000 tonnes of "humanitarian supplies", including medical equipment, baby food and sleeping bags, Russian media reported.
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".