Kiev says it will not allow Russian lorries carrying humanitarian aid to eastern Ukraine to cross the border unless the Red Cross is involved.
A convoy of almost 300 lorries, reportedly containing grain, sugar, baby food, medicine, electric power generators and sleeping bags, is heading for Ukraine's war-torn eastern regions of Donetsk and Lugansk.
But Ukrainian Prime Minister Arseniy Yatseniuk said Moscow was behaving in a cynical fashion after sending "terrorists" to foment a pro-Russian uprising in the east of the country.
"First they send tanks, Grad missiles and bandits who fire on Ukrainians, and then they send water and salt."
He said he did not exclude the possibility that the aid would be allowed through, but added that this would only happen with the assistance of the Red Cross.
The lorries left Moscow on Tuesday for the 600-mile journey to eastern Ukraine.
Humanitarian aid for those trapped by fighting there is needed, but there are fears Moscow, accused by Ukraine of arming pro-Russian separatists, could be using this as a ruse to annex disputed parts of the country.
Russia denied these claims, describing them as "absurd".
This week has seen the fiercest fighting of the conflict between Russia and Ukraine, with the number of people killed in the last fortnight rising to 2,000, according to the UN.
Pro-Russian separatists are currently losing ground to Ukrainian government forces.
The conflict erupted four months ago after Ukraine's pro-Russian president Viktor Yanukovych was driven from office following protests in Kiev.
State-controlled Russian television said fighting was taking place in the rebel stronghold of Donetsk and accused the Ukrainian military of indiscriminate shelling of civilian buildings.
Ukraine said on Tuesday the cargo would have to be unloaded at the border and transferred under Red Cross supervision to other vehicles.
12 August 2014
27 July 2014