* Convoy has been stuck at border for nearly a week
* Ukraine fears Russia using convoy to help rebels
* Moscow says wants to avert "humanitarian catastrophe" (Adds details, background)
By Dmitry Madorsky
BORDER CROSSING POINT DONETSK, Russia, Aug 21 (Reuters) - U krainian border guards began on Thursday to inspect a Russian truck convoy carrying aid earmarked for humanitarian relief in eastern Ukraine that has been stranded at the frontier between the two former Soviet republics for nearly a week.
Kiev believes the convoy of some 260 trucks, carrying water, food and medicines, could prove a Trojan horse for Russia to get weapons to pro-Russian separatists battling Ukrainian forces in the region - a notion that Moscow has dismissed as absurd.
"I can confirm that at 2:15 p.m. (1115 GMT) the Ukrainian side began border-customs formalities relating to the Russian humanitarian cargo," border guard spokesman Andriy Demchenko told Reuters.
Asked on whose territory the cargo was, he replied: "On the territory of the Russian border point."
It was not clear when the trucks would finally be authorised to enter Ukrainian territory, which at that border point is under rebel control. The rebels granted Kiev's border guards permission to access the crossing to check the trucks.
A Reuters witness saw 16 trucks move into territory beyond the Russian checkpoint, later followed by a second mini-convoy of 16 vehicles.
The aid is intended to alleviate a humanitarian crisis in the city of Luhansk, one of two big cities in Ukraine's Russian-speaking east which is being held by the rebels. The other is Donetsk, the region's main industrial hub.
The forces of the Western-backed Kiev government have been steadily gaining the upper hand over the separatists but fighting continues to rage in Donetsk, Luhansk and other urban centres across the Russian-speaking region.
Luhansk has been largely cut off by the conflict and is into its 19th day without water and regular supplies of electricity, which have hit mobile and landline phone connections. Only vital foodstuffs are on sale in the few shops remaining open.
Kiev and its Western allies accuse Moscow of supporting and arming the rebels. Moscow denies such allegations but has warned of a "humanitarian catastrophe" in eastern Ukraine.
Ukraine has insisted that the truck convoy comply with border inspections and other formalities before being allowed to cross into its territory under supervision by the Red Cross, which will be responsible for distributing the aid.
The Geneva-based International Committee of the Red Cross (ICRC) has sent 35 staff to help smooth the way for the Russian convoy and intends to accompany the Russian drivers and trucks with its own vehicles.
"We are ready to roll with this convoy, there has been a last-minute delay. We are hopeful that it will be resolved shortly," ICRC spokesman Ewan Watson told Reuters.
"Last-minute decisions from the Ukrainian side have delayed the process," he said, declining to elaborate.
The ICRC has begun delivering aid donated by the Ukrainian government to a number of towns in eastern Ukraine, including Starobilsk, Lysychansk and Syevyerodonetsk, with the help of the Ukrainian Red Cross.
"The distribution of goods such as fruit and vegetables has already reached over 20,000 displaced people in shelters and hospitals," the ICRC said, referring to the aid sent by Kiev.
The United Nations has put the death toll in the conflict at over 2,000, including civilians and combatants. That figure has nearly doubled since late July, when Ukrainian forces stepped up their offensive and the conflict spread to major urban areas.
Reporting by Dmitry Madorsky; Additional reporting by Stephanie Nebehay in Geneva and Natalia Zinets in Kiev; writing by Vladimir Soldatkin and Alessandra Prentice; Editing by Gareth Jones