Four lorries from a 280-strong Russian aid convoy have moved into the customs zone near Ukraine's border, a Ukrainian security official has said, according to BBC News.

But Andriy Lysenko said cargo checks by Ukrainian border guards would not start until Thursday, August 21 because Russia had not provided all the necessary papers.

The vehicles are due to continue into eastern Ukraine to deliver aid to civilians trapped by fighting. Ukraine is concerned the convoy could be used to help pro-Russian activists.

Russia has repeatedly stressed that the vehicles are carrying only food and medical aid to civilians trapped in the conflict zone, as heavy fighting continues between Ukrainian government troops and the activists.

The activists in the Donetsk and Luhansk regions declared independence from Kiev in April, following annexation of the Crimean peninsula from Ukraine in March.

More than 2,000 civilians and combatants have been killed since mid-April, when Ukraine sent its troops to oust the activists. Ukraine accuses Russia of arming the activists and sending Russian soldiers into eastern Ukraine - a claim denied by the Kremlin.

Ukrainian government officials and several Western states had expressed fears that the Russian convoy could be carrying arms destined for the activists.

The convoy has been parked near the town of Kamensk-Shakhtinsky for several days after setting out from near Moscow on 12 August and is said to be carrying 2,000 tonnes of aid.

The Red Cross said Ukrainian and Russian customs officials had agreed to inspect the lorries but the issue was complicated by the Izvaryne checkpoint being controlled by activists.

On Saturday, Ukraine's government declared the convoy "legal" but Red Cross officials said it had still not been given clearance because of some outstanding security issues.