Diplomatic efforts to douse a firestorm over claims Ukraine’s forces destroyed Russian military vehicles have intensified.

Moscow and Kiev’s foreign ministers are preparing for an urgent meeting with their French and German counterparts on Sunday after the United States blasted Russia for its “extremely dangerous” escalation of the crisis in Ukraine.

French President Francois Hollande meanwhile has urged Ukraine to show “restraint and good judgment” as it pushes on with a brutal offensive to oust insurgents after four months of fighting that has killed more than 2000 people.

The latest spike in tensions came after Ukrainian President Petro Poroshenko claimed his troops had blown up part of a small Russian military convoy that British media spotted breaching the porous frontier on Thursday.

Russia dismissed the claims as “fantasies”, its latest denial of persistent allegations from the West that it is arming the rebels.

Amid the international slanging match, 280 trucks packed with what Russia claims is humanitarian aid waited for a third day about 30 kilometres from a rebel-held border post as Moscow and Kiev haggled over letting it across.

The West and Kiev fear the convoy could be a “Trojan horse” to bolster the flagging pro-Kremlin rebellion in eastern Ukraine or provide Moscow with an excuse to send in the 20,000 troops NATO says it has amassed on the border.

Russia’s foreign ministry has repeatedly demanded in recent days that Kiev cease fire in order for the aid to reach residents of blighted cities in the east who have been stuck for days without water or power.

Witnesses at the border heard blasts from the Ukrainian side and saw Moscow’s military hardware rumble along Russian territory close to the frontier.

Ukraine says it has sent scores of border officials to the Russian side to scour aid cargo but insists they are waiting for permission from the Red Cross to start work.

A spokeswoman for the International Committee of the Red Cross – which is meant to be overseeing the operation – says no inspections have yet started as negotiations continue.

“There was a meeting this morning between the Ukrainians and the Russians. We did not participate,” said Galina Balzamova.

“We are still waiting for agreement” between them.”

While Russia has denied all claims it is funnelling weapons to the rebels, a top separatist leader claimed reinforcements trained across the border.

A fresh injection of firepower consisted of 150 pieces of hardware and 1200 personnel with “four months of training on Russian territory”, Alexander Zakharchenko, prime minister of the self-proclaimed Donetsk People’s Republic, said in a video posted on a rebel website.

“They have been brought in at the most crucial moment,” he said.

In Donetsk, the largest rebel bastion, the rumble of shelling could be heard Saturday as government troops tightened the vice around separatist fighters holed up there.