The stand-off came as violence worsened in eastern Ukraine, with the United Nations announcing that the death toll in the four-month conflict had doubled in the past two weeks to more than 2,000.
Government forces are seeking to press their advantage against the pro-Russian rebels, which has stoked fears that the aid convoy is part of a Russian plan to fend off the rebels' seemingly inevitable defeat.
"The level of Russian cynicism knows no bounds," said Arseniy Yatsenyuk, the Ukrainian prime minister.
"First they send tanks, Grad missiles and bandits who fire on Ukrainians – and then they send water and salt."
Arsen Avakov, Ukraine's interior minister, had said the "Putinist" convoy would not pass, calling it a "provocation by a cynical aggressor".
Russia responded by saying it had met Ukrainian demands for transparency and that Kiev was reneging on an agreement to deliver aid via the International Committee of the Red Cross (ICRC).
Spokesmen for the ICRC, which both Moscow and Kiev say must play a role in any aid delivery, said it was still waiting for a proper list of the convoy's cargo.
"We need much more specific information before we can begin to plan an operation like this," said Andre Loersch, an ICRC official based in Kiev.
Neither side disputes the need for aid in Luhansk, a city of 400,000 that has seen some of the fiercest fighting in the conflict.
The Red Cross has described the situation there as "dire," with civilians living under constant shelling, mains water and electricity cut off and food and medicine running low.
At least 2,086 people, including 20 children, have been killed in eastern Ukraine as of Aug 10, the United Nations said.
The "very conservative" figure has nearly doubled from 1,129 deaths counted on July 26, the office of the High Commissioner for Human Rights said.
The convoy, which departed from an army base near Moscow on Tuesday, appeared to vanish yesterday, adding to confusion surrounding the route and purpose of the cargo.
Last seen in the city of Voronezh, 300 miles south of Moscow, on Tuesday, the convoy never reached the border near the Russian city of Belgorod that the Russian foreign ministry said it would use.
Rumours that the convoy had headed south from Voronezh to cross the border directly into rebel-held territory – a move that would be viewed by Kiev as tantamount to a declaration of war – could not be substantiated.
Russian state television initially reported the convoy to be en route to Belgorod.
But journalists waiting at the Pletenivka checkpoint, on the Ukrainian side of the border where Russia had said the convoy would cross, saw no sign of the vehicles.
Guards at the post indicated that the convoy was expected but there was no chance of it arriving yesterday.
They said they did not know the reason for the hold-up.
Vladimir Putin's spokesman yesterday said the convoy was "on the move", although reporters in Voronezh said it had never been seen leaving the airbase where the lorries spent the night.
In an apparent breakthrough last night, Ukraine said it could allow the Russian aid convoy to cross the border "in order to prevent a large-scale invasion".
"Our customs officers, border guards and the OSCE [Organisation for Security and Cooperation in Europe] can scan the goods on the Russian-Ukrainian border," he added.
The convoy would then proceed to rebel-held territory, where the Red Cross would take over distribution of aid in Luhansk, he said.
But with the route and details, or even the existence, of an agreement with the Kremlin unclear, it remains uncertain whether the apparent breakthrough will result in aid reaching residents in Luhansk.
Source: The Telegraph