KIEV, Ukraine — Ukraine's crisis entered dangerous new territory Saturday with Kiev claiming its forces destroyed a Russian military convoy, while the U.S. warned Moscow over its "provocative" efforts at destabilization.

Moscow and Kiev's top diplomats agreed to an urgent meeting with their French and German counterparts Sunday in an attempt to calm the soaring tensions, as wrangling continued over the fate of a mammoth Russian aid convoy parked up close to the Ukrainian frontier.

The U.S. National Security Council warned of an "extremely dangerous and provocative" escalation in the crisis by Russia after Ukrainian President Petro Poroshenko claimed his troops had blown up part of small Russian army convoy — separate from the humanitarian convoy — that breached the border.

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

But the European Union piled the pressure on Moscow by insisting it "put an immediate stop to any form of border hostilities, in particular to the flow of arms, military advisers and armed personnel into the conflict region."

The humanitarian situation in war-torn east Ukraine has deteriorated still further as four months of fierce clashes that have killed over 2,000 people and left people without water and basic services raged on.

Moscow has sent nearly 300 trucks of what it claims is humanitarian aid to eastern Ukraine. The West and Kiev fear it could be a "Trojan horse" of arms and possibly even soldiers bound for the restive east of the country.

The Russian convoy has been parked since Thursday about 20 miles from the border with Ukraine's Lugansk region, as Kiev and Moscow continued to haggle over how it would cross into rebel-held territory.

A Red Cross spokeswoman in the border zone told AFP Saturday that so far the organization has not inspected any of the trucks, waiting for both sides to give a green light.

"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, she added.

The United States warned Moscow against sending "vehicles, person, or cargo of any kind into Ukraine, under any pretext" without Kiev's permission. 

Russia has denied sending troops or weapons to eastern Ukraine, and slammed the claims of the latest military convoy as a "phantom" of Ukraine's imagination.

A rebel leader of the Donetsk People's Republic, one of the breakaway territories where pro-Russian insurgents are fighting Kiev's army, said however that his troops have had a fresh injection of fighters trained across the border.

The rebel reinforcements consist of 150 pieces of hardware and 1,200 personnel "who have received four month of training on Russian territory," Alexander Zakharchenko, he said at a meeting of rebel leadership Friday, videoed by insurgent website Novorossiya Online.

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

In Donetsk, the largest city in the east controlled by the rebels, continuous shelling could be heard by AFP correspondents overnight Saturday.

Lugansk has been the city hardest hit by the shelling. Human Rights Watch said in its most recent dispatch from the city that the focus on aid should not "eclipse the crucial need for all parties to avoid harming civilians with explosive weapons attacks."

Those fleeing the stricken city told the rights group that the city was cut off from electricity, gas, or cell phone coverage, and it was difficult to find drinking water and food.

The United Nations says over 285,000 people have fled the fighting in the east.

A flurry of diplomacy following the claims by Kiev of destruction of the military vehicles from Russia yielded an agreement by Ukraine, Russia, France and Germany to gather for a meeting of foreign ministers in Berlin Sunday.

Germany's Frank-Walter Steinmeier told Bild newspaper Saturday that he hoped the talks would help "put an end to violent fighting" in eastern Ukraine and provide the territories with "urgent and necessary aid."

The four-way crisis meeting was agreed Friday between Russia's influential chief-of-staff in the Kremlin, Sergei Ivanov, and Ukrainian counterpart Boris Lozhkin, who met in Russia's Black Sea resort of Sochi.

Ukraine's Foreign Minister Pavlo Klimkin said that the sides "need to talk" as tensions spiraled.

However few details surfaced as of Saturday about the talks' specific agenda or even what time it would take place.