Diplomatic efforts to douse a firestorm over claims Ukraine's forces destroyed Russian military vehicles have intensified.
French President Francois Hollande meanwhile has urged Ukraine to show "restraint and good judgment" as it pushes on with a brutal offensive to oust insurgents after four months of fighting that has killed more than 2000 people.
280 trucks packed with what Russia claims is humanitarian aid waited for a third day about 30 kilometres from a rebel-held border post as Moscow and Kiev haggled over letting it across.
The US National Security Council warned of an "extremely dangerous and provocative" escalation in the crisis by Russia after Ukrainian President Petro Poroshenko claimed his troops had blown up part of small Russian army convoy - separate from the humanitarian convoy - that breached the border.
Russia dismissed the claim as "fantasies," its latest denial of persistent allegations from the West that it is arming the rebels.
Moscow has sent nearly 300 trucks of what it claims is humanitarian aid to eastern Ukraine. The West and Kiev fear it could be a "Trojan horse" of arms and possibly even soldiers bound for the restive east of the country.
The Russian convoy has been parked since Thursday about 30 kilometres from the border with Ukraine's Lugansk region, as Kiev and Moscow continued to haggle over how it would cross into rebel-held territory.
A Red Cross spokeswoman in the border zone said on Saturday that so far the organisation has not inspected any of the trucks, waiting for both sides to give a green light.
The United States warned Moscow against sending "vehicles, person, or cargo of any kind into Ukraine, under any pretext" without Kiev's permission.
Russia has denied sending troops or weapons to eastern Ukraine, and slammed the claims of the latest military convoy as a "phantom" of Ukraine's imagination.
A rebel leader of the Donetsk People's Republic, one of the breakaway territories where pro-Russian insurgents are fighting Kiev's army, said however that his troops have had a fresh injection of fighters trained across the border.
The rebel reinforcements consist of 150 pieces of hardware and 1,200 personnel "who have received four month of training on Russian territory," Alexander Zakharchenko, he said at a meeting of rebel leadership Friday, videoed by insurgent website Novorossiya Online.
"They have been brought in at the most crucial moment," he said.
In Donetsk, the largest city in the east controlled by the rebels, continuous shelling could be heard overnight Saturday.
Lugansk has been the city hardest hit by the shelling. Human Rights Watch said in its most recent dispatch from the city that the focus on aid should not "eclipse the crucial need for all parties to avoid harming civilians with explosive weapons attacks".
Those fleeing the stricken city told the rights group that the city was cut off from electricity, gas, or mobile phone coverage, and it was difficult to find drinking water and food.
The United Nations says over 285,000 people have fled the fighting in the east.
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