Pro-Russian rebels on Sunday shot down a Ukrainian fighter jet ahead of urgent talks between Kiev and Moscow's top diplomats to defuse tensions over fighting in the east of the ex-Soviet nation.

The foreign ministers of Russia, Ukraine, Germany and France headed for a meeting in Berlin, after Western leaders scrambled to dampen a firestorm over claims that Ukrainian forces had destroyed Russian military vehicles.

Ukraine's military told AFP its MiG-29 warplane was blown out of the sky as it carried out "an assignment to eliminate a large group of terrorists" in the strife-torn Lugansk region. The pilot managed to parachute to safety, it said.

Authorities in the main rebel city of Donetsk said shelling killed 10 civilians in 24 hours as government forces pressed on with an offensive to oust separatists after four months of fighting that has left over 2,100 people dead and pushed the region to the brink of a humanitarian catastrophe.

Germany meanwhile demanded that Moscow clarify boasts by a rebel leader in the besieged city that he had recently received hundreds of fighters trained in Russia to bolster the flagging rebellion.

Fresh weekend clashes will serve only to fray nerves further ahead of Sunday's meeting between Russian Foreign Minister Sergei Lavrov, Ukraine's Foreign Minister Pavlo Klimkin, and their French and German counterparts.

"Flying to Berlin. The talks will not be easy. It is important to stop the flow of weapons and mercenaries from Russia," Klimkin wrote on Twitter Sunday.

Progress on aid?

French President Francois Hollande called for Ukraine to show "restraint and good judgement" in its military operations after Kiev's boasts that it had destroyed part of a small military convoy from Russia ramped up the stakes.

He also suggested the talks could pave the way for a face-to-face encounter between the Russian and Ukrainian heads of state.

Russia had dismissed the incursion claims as "fantasies", but resisted the urge to strike back, as it again denied the persistent allegations from the West that it is arming the rebels.

Meanwhile, the fate of a mammoth Russian aid convoy parked up near Ukraine's border since Thursday remained uncertain despite both sides appearing to edge closer to a deal to let it across the volatile border.

The Red Cross told AFP its officials had arrived at an area where some 300 Russian trucks are waiting but that official inspections of the cargo were yet to begin.

AFP journalists later saw a group of 16 trucks head in the direction of the crossing.

The West and Kiev fear the convoy could be a "Trojan horse" to help the rebels in eastern Ukraine, or provide Moscow with an excuse to send in the 20,000 troops that NATO says it has massed on the border.

The International Committee of the Red Cross, which is overseeing the aid delivery, has said Russia and Ukraine agreed on procedures to check the cargo but "security guarantees" are still needed on how the vehicles could cross rebel-held territory.

Kiev recognised the "legality" of the humanitarian convoy in a statement published on the government web site, moving closer to giving the green light for the trucks to enter its territory.

Ukraine President Petro Poroshenko told US Vice President Joe Biden Saturday that the separatists had yet to grant safe passage for the aid.

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

Rebel reinforcements

German Chancellor Angela Merkel called on Moscow to "clarify" claims from a top separatist leader that troop reinforcements trained across the border had arrived to prop up the ailing insurgency.

Alexander Zakharchenko, "prime minister" of the self-proclaimed Donetsk People's Republic, said in a video posted online Saturday that rebel forces had received 1,200 personnel "who have received four months of training on Russian territory" as well as 150 items of military hardware, including tanks.

Continuing fighting worsened the humanitarian crisis, with residents fleeing Lugansk telling Human Rights Watch that there was no gas and cell phone coverage in the city and it was difficult finding drinking water and food.

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