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Sergei Grits, Associated Press
Pro-Russian rebels stand at a block-post on the outskirts of Donetsk seen through the bullet riddled windshield of the bus, eastern Ukraine, Wednesday, Aug. 13, 2014, where at least 12 militiamen fighting alongside government troops against pro-Russian separatist rebels have been killed in an ambush, a spokesman for their radical nationalist movement said Wednesday.

VORONEZH, Russia — Hundreds of Russian trucks carrying aid intended for rebel-held eastern Ukraine remained parked Wednesday in the southern city of Voronezh, their fate shrouded in mystery as Ukraine accused Moscow of plotting to use them as a cover for invasion.

Fighting between government troops and pro-Russian separatists increased as the U.N.'s human rights office released figures showing the number of people killed in eastern Ukraine appears to have doubled in the last two weeks to more than 2,000.

Other than what appeared to be a few supply runs, the roughly 262 vehicles in the convoy lay idle at a military base in the southern city of Voronezh well into the afternoon, one day after making the 400-mile (650-kilometer) drive from a town outside Moscow.

Ukraine and Russia on Tuesday tentatively agreed that the aid would be delivered to a government-controlled crossing in Ukraine's Kharkiv region, which hasn't been touched by the months of fighting that have wracked neighboring regions. The cargo would then have to be inspected by the International Committee of the Red Cross.

But accord has soured into acrimony with the spokesman for Ukrainian President Poroshenko accusing Moscow of possibly planning a "direct invasion of Ukrainian territory under the guise of delivering humanitarian aid."

Andriy Lysenko, a spokesman for Ukraine's National Security and Defense Council, said that he "had information" that the convoy won't go through Kharkiv, but that "nobody knows where it will go."

That leaves the option for the convoy to go through a portion of the border further south that is under the control of the armed pro-Russian separatist rebels that the government has been battling for the last four months. This scenario would all but certainly not involve the Red Cross and is viewed with profound hostility by the Ukrainian government.

Lysenko said that any deliveries of aid "that don't have the mandate of the Red Cross ... are taken as aggressive forces and the response will be adequate to that."

Russian President Vladimir Putin's spokesman, Dmitry Peskov, insisted that the aid convoy was on the move inside Russia, but declined to comment on the route. He said the operation was proceeding in full cooperation with the Red Cross.

But Red Cross officials in Ukraine said they have been left in the dark about the whereabouts of the Russian aid.

"The final route is not known. Even at the moment I am trying to find out where the convoy is," said Andre Loersch, a spokesman for the ICRC mission in Ukraine.

Amid the tensions, Putin traveled to Crimea, a Black Sea peninsula that Russia seized from Ukraine in March, where he chaired a session of his Security Council. A meeting with Putin's entire Cabinet and most Russian lawmakers has been scheduled for Thursday.

The estimated 2,000 metric tons of aid, which reportedly includes goods ranging from baby food to portable generators, is intended for civilians in the Luhansk region, the scene of some of the fiercest fighting between government troops and pro-Russian separatists. The regional capital of Luhansk has had no electricity for 11 days and only the most essential goods are available, city authorities said.

A spokeswoman for the U.N.'s human rights office, Cecile Pouilly, said in a statement Tuesday that the U.N.'s "very conservative estimates" show the overall death toll has risen to at least 2,086 people as of Aug. 10 from 1,129 on July 26.

Pouilly said at least 4,953 others have been wounded in the fighting since mid-April.

Intense fighting was still taking place in the main rebel-held city of Donetsk. At least 12 militiamen fighting alongside government troops were killed in an ambush outside the besieged city, a spokesman for their radical nationalist movement said Wednesday. Inside the city, at least three people were killed overnight Tuesday as the government intensified its shelling.

Artem Skoropatsky said the Right Sector volunteer fighters were shot dead while traveling on a bus and many others on the bus were wounded and taken captive. He did not know how many.

"There is a suspicion that the wounded will be treated very harshly and could be shot," he said.

Right Sector played a marginal if highly visible role in the protests that culminated in the overthrow of President Viktor Yanukovych in February. Its far-right nationalist affiliations have made it a target of lurid reporting in Russia state media, which has sought to cast the post-Yanukovych government as extremists.

Lysenko, the National Security and Defense Council spokesman, said Wednesday that 11 servicemen were killed in the previous day's fighting, but he could not immediately say whether that figure included the Right Sector militiamen.

Government troops have laid siege to Donetsk and nearby rebel holdings in their push to quash the pro-Russian insurgency. They have largely refrained from street-to-street fighting, favoring often inaccurate rocket fire.

Residents said the intermittent artillery barrage lasted around two hours. City authorities said 10 residential buildings and the wing of a hospital were struck.

Associated Press reporters saw two bodies lying in a street Wednesday morning in Donetsk's southwestern Petrovsky district. The city government reported three deaths.

The shelling has damaged power plants and gas pipelines, leaving large parts of the city without electricity or gas.

The fighting in eastern Ukraine began in April, a month after Putin annexed Crimea.

Grits reported from Donetsk, Ukraine. John Heilprin in Geneva, Yuras Karmanau in Donetsk and Peter Leonard in Kiev, Natalya Vasilyeva in Sevastopol, Crimea, contributed to this report.