Ukraine denounced Russia's dispatch of a humanitarian aid convoy now advancing towards the border as an act of unbounded cynicism serving pro-Russian separatists, and the UN said the death toll in fighting had doubled in the last two weeks to over 2,000.
Kiev said the trucks would not be allowed to pass.
"First they send tanks, Grad missiles and bandits who fire on Ukrainians and then they send water and salt," Prime Minister Arseny Yatseniuk said at a government meeting on Wednesday.
The comments reflected suspicions in Kiev and Western capitals that passage of the convoy onto Ukrainian soil could turn into a covert military action to help pro-Russian separatists now losing ground to government forces.
The convoy of 280 heavy trucks rumbled out of Moscow region on Tuesday and traveled some 500 km (300 miles) to the southwestern Russian town of Voronezh.
There it stopped at an air base behind high fences, according to a Reuters reporter at the scene.
Several people who entered the airbase and spoke to Reuters on condition of anonymity said dozens of trucks were still parked at the airbase. It was not clear whether the Voronezh convoy was the only one travelling towards Ukraine.
"The journey isn't short of course," one lorry driver interviewed on Russian Rossiya-24 television commented.
"How can I put it? It's pretty difficult. But how could we not help our Slavic brothers? We are all for it."
There was no discussion on Russian television of any military intervention and Moscow has insisted throughout that it has enlisted Ukrainian cooperation for the operation.
The last few weeks has seen signficant government successes against rebels who have abandoned a string of towns under heavy fire. Kiev says rebel leaders, some of whom are Russians and who portray the Kiev government as nationalist and seek union with Russia, are receiving arms from Russia, something Moscow denies.
U.N. human rights spokeswoman Cecile Pouilly said the estimated death toll had risen to 2,086 as of Aug. 10 from 1,129 on July 26. The figures included Ukrainian soldiers, rebel groups and civilians, but were "very conservative estimates".
"This corresponds to a clear escalating trend," she told Reuters in response to a query.
It was not clear to what extent the figures reflected escalation of the fighting, which erupted four months ago after a pro-Russian president was driven from office by protests, or delays in gathering data from government and local agencies.
State-controlled Russian television presented a picture of fierce battles around the rebel stronghold of Donetsk, accusing the Ukrainian military of indiscriminate shelling of rocketing of civilian buildings. Residents interviewed said they were being bombed everyday and hiding in cellars.
The approach of the convoy may present Kiev with a dilemma.
Clearly, Ukraine fears it could become the focus of tension and conflict once on its soil and provide pretext for a Russian armed incursion. At the same time it does not want to seem to be blocking aid and providing the same moral arguments to Moscow.
"The level of Russian cynicism knows no bounds," said Prime Minister Yatseniuk, outlining the situation at a government meeting.
Interior Minister Arsen Avakov said on his Facebook page: "No Putin 'humanitarian convoy' will be allowed across the territory of Kharkiv region.
The provocation by a cynical aggressor will not be allowed on our territory."
Yatseniuk reiterated that any kind of humanitarian aid from the outside had to be organised under the auspices of the International Committee of the Red Cross. Russia has said it would agree to this.
It was not immediately clear if Kiev was rejecting outright the Russian aid or simply refusing to allow the Russian trucks onto Ukrainian territory.
Ukraine said on Tuesday that the cargo would have to be unloaded from Russian trucks at the border and transferred under international Red Cross aegis onto other vehicles. The European Union (EU) said the contents would have to be scrutinised.
"We are currently waiting for an exact description of the goods which are there. This is a key moment to work out the options for delivery and distribution of aid. The final route is not known," a Ukrainian Red Cross spokesman said in Kiev.
Kiev accuses Russia of supporting and arming the rebels - who now appear to be on the verge of defeat by government forces - with tanks, missiles and other weapons. Moscow denies this.
Four months of fighting in the east has produced a humanitarian crisis in parts of eastern Ukraine. People in the main cities of Donetsk and Luhansk, on the border with Russia, are suffering acute shortages of water, food and electricity.
Yatseniuk said the Kiev government had received 6 million dollars from its Western partners which would be used to alleviate conditions in distressed areas.
"We as the government of Ukraine are sending vitally needed goods to all the liberated territories," Yatseniuk said, meaning those places which had been recaptured from the rebels.
"We, as a state, are looking after, and are capable of looking after, our citizens," he said.
Russia says the convoy is carrying about 2,000 tonnes of water, baby foods and other goods.
The Russian RIA agency said it had left in the morning for the town of Belgorod where it would cross into Ukraine's Kharkiv region. A local journalist in Belgorod said he had heard it would arrive there from 3 p.m.-4 p.m. (1100-1200 GMT).
Yatseniuk, speaking to a government meeting, said: "The level of Russian cynicism knows no bounds. First they send tanks, Grad missiles and bandits who fire on Ukrainians and then they send water and salt."
Interior Minister Arsen Avakov said on his Facebook page: "No Putin 'humanitarian convoy' will be allowed across the territory of Kharkiv region. The provocation by a cynical aggressor will not be allowed."