The unilateral move sharply raised the stakes in eastern Ukraine, for any attack on the convoy could draw the Russian military directly into the conflict between the Ukrainian government and the separatist rebels in the east. Ukraine has long accused Russia of supporting and arming the rebels, a charge that Russia denies.
In the past few days, Ukraine says its troops have recaptured significant parts of Luhansk, the second-largest rebel city, and suspicions were running high that Moscow's humanitarian operation may instead be aimed at halting Kiev's military momentum. Fierce fighting has been reported this week both around Luhansk and the largest rebel-held city, Donetsk, with dozens of casualties.
Speaking on national television, Prime Minister Arseniy Yastenyuk declared that Russia's plan in sending half-empty trucks into Ukraine was not to deliver aid but to create a provocation by attacking the convoy itself, thus arranging a "provocation."
Ukrainian security services chief Valentyn Nalyvaichenko called the convoy a "direct invasion."
The white-tarped semis, which Russia says are carrying food, water, generators and sleeping bags, are intended for civilians in the hard-hit city of Luhansk, where government forces are besieging pro-Russian separatists. The city has seen weeks of heavy shelling that has cut off power, water and phone lines and left food supplies scarce.
The swiftness with which Russia set the mission into motion last week and the lack of direct involvement from the international community immediately raised questions about Moscow's intentions. AP journalists following the convoy across country roads heard the trucks' contents rattling and sliding Friday, confirming that many vehicles were only partially loaded.
Nalyvaichenko, speaking to reporters in the Ukrainian capital of Kiev, said the men driving the trucks into Ukraine were Russian military personnel "trained to drive combat vehicles, tanks and artillery." The half-empty aid trucks would be used to transport weapons to rebels and spirit away the bodies of Russian fighters killed in eastern Ukraine, he said.
Ukraine's presidential administration said Kiev authorized the entrance of only 35 trucks. But the number of Russian vehicles entering the country through a rebel-held border point Friday was clearly way beyond that amount.
"The Russian side has decided to act," the Russian Foreign Ministry said in a statement. "It is no longer possible to tolerate this lawlessness, outright lies and inability to reach agreements ... we are warning against any attempts to thwart this purely humanitarian mission."
Although Luhansk is only 12 miles (20 kilometers) from the border, the Russian convoy took a meandering route, apparently in an effort to avoid areas controlled by Ukrainian troops.
Shortly after leaving the rebel-held border town of Izvaryne, the convoy turned off of the main highway to Luhansk and headed north on a country road. Rolling on small roads greatly slowed the trucks' progress, turning what would in peacetime take roughly two hours into a daylong haul.
Rebel forces took advantage of Ukraine's promise not to shell the convoy to drive on the same country road as the aid trucks. Around lunchtime, around 20 green military supply vehicles — flatbed trucks and fuel tankers — were seen traveling in the opposite direction. Other smaller rebel vehicles could be seen driving around.
The convoy moved along village roads hugging the Russian border, which is marked by the winding Seversky Donets River. In the village of Davydo-Mykilske, less than one kilometer (half a mile) west of the border, AP reporters saw three rebel tanks, dozens of militiamen and several armored personnel carriers.
The trucks from Moscow had been stranded in a customs zone for more than a week since reaching the border. The Russian Foreign Ministry voiced increasing frustration at what it said were Kiev's efforts to stall its delivery, while Ukraine demanded that the trucks enter through a government-controlled border post so it could check their contents.
The Russian Foreign Ministry had accused the government in Kiev of shelling areas the convoy would have to pass through, making its travel impossible.
"There is increasingly a sense that the Ukrainian leaders are deliberately dragging out the delivery of the humanitarian load until there is a situation in which there will no longer be anyone left to help," it said Friday in a statement.
Ukraine's Foreign Ministry retorted with a statement accusing Russia of "ignoring international rules, procedures and agreements that have been reached."
Last week, after the Russian aid convoy left Moscow, Ukraine mounted its own humanitarian operation for those affected by fighting in the east. The rebels have said, however, they will not allow that material to enter their territory.