Ukraine's military said its MiG-29 warplane had been shot down as it carried out "an assignment to eliminate a large group of terrorists" in the Luhansk region.
The pilot managed to parachute to safety, it said.
Authorities in the main rebel city of Donetsk said shelling had killed 10 civilians in 24 hours as government forces pressed on with an offensive to oust separatists.
Meanwhile, Germany 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.
Russia's foreign minister, Sergei Lavrov, Ukraine's foreign minister, Pavlo Klimkin, and their French and German counterparts are due to meet in Berlin.
Klimkin tweeted: "Flying to Berlin. The talks will not be easy. It is important to stop the flow of weapons and mercenaries from Russia."
The French president, François Hollande, called for Ukraine to show "restraint and good judgment" in its military operations, after boasts by Kiev that it had destroyed part of a small military convoy from Russia.
He 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.
The fate of a Russian aid convoy parked up near the border since Thursday remained uncertain despite both sides appearing to edge closer to a deal to let it into Ukraine.
The Red Cross said its officials had arrived at an area where 300 Russian trucks were waiting, but 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 that 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 have agreed on procedures to check the cargo, but "security guarantees" are still needed on how the vehicles can cross rebel-held territory.
Kiev recognised the "legality" of the humanitarian convoy in a statement published on the government website, moving closer to giving the green light for the trucks to enter its territory.
Ukraine's president, Petro Poroshenko, told the US vice-president, Joe Biden, on 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.
Source: The Guardian