Raising the stakes in Ukraine's conflict, a Russian aid convoy of more than 200 trucks pushed up to the border on Thursday but then stopped, provocatively poised to cross into rebel-held territory.
The Ukrainian government threatened to use all means available to block the convoy if the Red Cross was not allowed to inspect the cargo. Such an inspection would ease concerns that Russia could use the aid shipment as cover for a military incursion in support of the separatists, who have come under growing pressure from government troops.
The United States has warned Russia that it needs to secure Ukraine's permission for the convoy to enter.
"We've made that very clear to the Russians that they should not move these trucks in, without taking all of the steps the Ukrainian government has outlined," U.S. State Department spokeswoman Marie Harf said Thursday.
Amid the tensions surrounding the convoy, European Commission chief Jose Manuel Barroso called Russian and Ukrainian leaders to arrange three-way consultations on ways to de-escalate the crisis.
Ukraine announced it was organizing its own aid shipment to the war-ravaged separatist region of Lugansk.
Complicating the dispute over the duelling missions, Ukraine said Thursday it has gained control over a key town near Lugansk city, thereby giving it the means to block the presumed route that the Russian convoy would take to the city.
The town, Novosvitlivka, lies about 40 kilometres from the border, so if the Russian trucks did enter the country, they potentially could unload somewhere other than the city itself.
The Russian convoy set out Thursday morning from a military depot in the southern Russian city of Voronezh, where it had been parked since late Tuesday. Moscow says the convoy has 262 vehicles, including about 200 trucks carrying aid.
The white-tarped trucks, some flying the red flag of Moscow and escorted by military vehicles, drove down a winding highway through sunflower fields and then turned west toward the rebel-held border crossing of Izvaryne.
But about 30 kilometres from the border, the trucks pulled off and parked in a large field where dozens of beige tents had been set up. Drivers in matching khaki shorts and shirts piled out and appeared to be stopping for the night.
The route suggested Russia has decided not to abide by a tentative agreement to deliver aid to a government-controlled border checkpoint in the Kharkiv region, where it could more easily be inspected by Ukraine and the Red Cross.
Taking Novosvitlivka "disrupted the last opportunity for movement between Lugansk and other territories controlled by Russian mercenaries," Ukrainian security council spokesman Andriy Lysenko told reporters.
Mr. Lysenko also said that if the Russians refused to let the Red Cross inspect its cargo "the movement of the convoy will be blocked with all the forces available."
Ukraine suspects the convoy could be a pretext for a Russian military invasion or further support for the pro-Russian rebels it has been fighting since April.
While the disputed aid convoy stopped short of the border, The Guardian newspaper reported seeing a separate convoy of Russian military vehicles crossing into Ukraine late Thursday.
The paper said a column of 23 armoured personnel carriers with Russian military plates crossed through a gap in barbed wire fence into Ukrainian territory. Ukraine and The West have long accused Russia of carrying out such incursions to help arm the rebels, claims that Moscow denies. It was impossible to independently verify the report.
After a clumsy and ineffectual start, Ukraine's forces have taken back much of the territory once held by rebels.
As the circle around the separatists tightens, two of their top figures have resigned in the past week. On Thursday, the rebel Donetsk People's Republic said its defence minister Igor Girkin had resigned.
Both Mr. Girkin and former rebel prime minister Alexander Borodai, who was replaced last week, are Russians and both were replaced by Ukrainians. Those moves could indicate an attempt by the separatists to distance themselves from allegations by Kiev and the West that Russia supports or directs the insurgency, charges that Russia denies.
The Russian aid convoy had been parked at a military depot in the southern Russian city of Voronezh amid disagreement over how and where the aid could be delivered to eastern Ukraine.
Moscow has insisted it co-ordinated the dispatch of the goods – which it says range from baby food and canned meat to portable generators and sleeping bags – with the International Committee of the Red Cross (ICRC).
ICRC spokeswoman Anastasia Isyuk said talks were continuing but she could not confirm where the Russian convoy was headed.
"The plans keep changing, the discussions are going ahead and we will not confirm for sure until we know an agreement has been reached," Ms. Isyuk said in Geneva.
Ukrainian Infrastructure Minister Maxim Burbak said three convoys totalling 75 trucks were transporting 800 tonnes of humanitarian aid – including grain, sugar and canned food – from Kiev and the cities of Kharkiv and Dnipropetrovsk. Their eventual destination was Lugansk, he said.
Ukrainian forces have stepped up efforts to dislodge the separatists from their last strongholds in Donetsk and Lugansk and there was more heavy shelling overnight.
In Washington, Ms. Harf said that the U.S. has "stressed the importance of showing restraint to minimize" civilian casualties.
The sounds of artillery fire and blasts could be heard all over Donetsk on Thursday. Shells hit two shopping complexes, city authorities said, warning citizens to stay off the streets.
Valentina Smirnova, a resident of Donetsk, cleaned up broken glass and rubble Thursday in her damaged kitchen.
"My son left and now I am staying with my daughter. I don't know what to do afterward. Where should I run to after that? Please tell me!" she said, tears welling up.
The UN's human-rights office in Geneva says the death toll in eastern Ukraine has nearly doubled in the last two weeks – rising to at least 2,086 as of Aug. 10, up from 1,129 on July 26.