KAMENSK-SHAKHTINSKY, Russia (AFP) - Ukrainian officials were preparing to inspect a massive Russian "aid" convoy bound for the conflict-torn east on Friday after Russian armoured vehicles crossed the border, fuelling fears Moscow is trying to bolster the unravelling insurgency.

The Ukrainian military had announced that checks had begun on the near 300-truck convoy but later said only that 59 border and customs officials had arrived at a Russian border post to prepare to carry out the inspections.

Military spokesman Leonid Matyukhin told AFP that after the inspections the trucks would head to the rebel-held bastion of Lugansk, where local officials have warned of a humanitarian crisis with shortages of food, water and power.

"As for now the work by the Ukrainian officials has not begun as they do not yet have the documents from the Red Cross giving them permission," the military said in statement.

Ukraine and Western powers suspect that Moscow could try to use the convoy as a "Trojan horse" bringing military help to pro-Russian insurgents, who have been rapidly losing ground to government troops in the industrial east.

On Friday, Ukrainian authorities confirmed British media reports that a small number of Russian armoured vehicles had crossed into Ukraine, not far from where the aid convoy was parked near the Russian town of Kamensk-Shakhtinsky.

"Intelligence has confirmed that a column of APCs and Ural military lorries has crossed the border and entered Ukrainian territory... intelligence also confirms that there were Russian number plates and insignia on the military hardware," the Interfax Ukraine news agency quoted the military as saying.

AFP journalists also saw a column of about a dozen troop carriers on the road heading to the Russian border post of Donetsk.

Ukraine and the West have often accused Moscow of sending armour across the border to help the pro-Kremlin separatists who launched an insurgency against Kiev in April, emboldened by Russia's annexation of Crimea in March.

But the rebellion was showing signs of unravelling after four months of fighting that has left more than 2,000 people dead including children and sent around 285,000 fleeing their homes.

The top rebel military chief Igor Strelkov and another key commander announced Thursday they were quitting after Ukraine's military said it had completely surrounded Lugansk, cutting all links to the border with Russia.

The death toll from the conflict continues to climb, with shelling in the main besieged rebel stronghold of Donetsk killing 11 civilians over the past 24 hours, local authorities said Friday.

Britain's The Daily Telegraph reported that it had "witnessed a column of vehicles including both armoured personal carriers and soft-skinned lorries crossing into Ukraine".

The Guardian newspaper also said a column of 23 armoured personnel carriers and other vehicles had crossed near the Russian border town of Donetsk.

Kiev is deeply suspicious about Russia's real intentions behind the aid mission.

But Moscow denies allegations it plans to send in troops or equipment to the rebels, and insists the operation was coordinated with the International Committee of the Red Cross.

On Thursday, the Russian foreign ministry called for all sides to accept an "urgent" ceasefire, saying the situation in the blighted east was "extremely serious".

Washington, while reiterating its support for Kiev, also urged its ally to exercise "restraint" and keep civilian casualties to a minimum after intense shelling in Lugansk and Donetsk left over 25 people dead.

"We call on the Ukrainians to take every step to avoid the local population as they try to free the city from the separatists," State Department spokeswoman Marie Harf told reporters.

Ukraine's military said Friday that troops were pushing on with their offensive by retaking three small towns, while fierce clashes were also reported on the outskirts of Lugansk.

Russian media reports say the convoy of white-tarpaulin-covered trucks, which left the Moscow region on Tuesday, was carrying more than 1,800 tonnes of supplies including medical equipment, baby food, sleeping bags and electric generators.

It was unclear however when it might be allowed into Ukraine.

Kiev had initially said it would not allow the trucks to enter but later suggested the aid could come in if it was inspected by Ukrainian border guards and international monitors.

Ukraine has also dispatched its own aid convoys to the east - 75 lorries with 800 tonnes of aid.