Kiev: Kiev forces battled into a key rebel bastion on Sunday as Moscow denied fresh claims that Russian rocket launchers had crossed over into Ukraine to bolster the separatists' flagging insurgency.
In Berlin meanwhile the foreign ministers of Russia, Ukraine, Germany and France gathered for a crisis meeting, and Germany's Frank-Walter Steinmeier said talks would be "all about finding a roadmap toward a sustainable ceasefire".
Kiev's military said it hoisted the national flag over a district police station in a northeast suburb of the second-largest rebel bastion of Lugansk after a fierce battle with pro-Russian separatists on Saturday.
A push into the city limits of the stricken 420,000-strong industrial hub would be a major breakthrough for government forces after four months of fighting that has claimed more than 2,100 lives and brought the region to the brink of a humanitarian catastrophe.
Around the region fierce clashes continued between government and rebels, with Ukraine's military saying a MiG fighter jet was shot down not far from Lugansk and the pilot parachuted
Lugansk has been the scene of some of the heaviest clashes, and local authorities said that power and water were not working for a 15th day running in the increasingly deserted city.
The United Nations says more than 285,000 people have fled the fighting in the east.
Authorities in the besieged main rebel city of Donetsk said shelling killed 10 civilians and wounded eight in 24 hours as government forces tightened the vice on rebels hunkered down there.
Ukraine also ramped up the stakes before the talks in Germany by alleging another military convoy including three Grad rocket systems crossed over from Russia.
The fresh claims come as a furore still swirls over Kiev's earlier boasts that it destroyed part of a Russian armoured convoy that breached the frontier on Thursday.
Ukraine's Foreign Minister Pavlo Klimkin said Sunday's talks with his Russian counterpart Sergei Lavrov would "not be easy" as Germany also demanded that Moscow clarify rebel claims that they had received hundreds of fighters trained in Russia to shore up their insurgency.
"It is important to stop the flow of weapons and mercenaries from Russia," Klimkin wrote on Twitter Sunday.
A Kremlin spokesman denied Moscow had sent "equipment" across the border, its latest dismissal of persistent allegations by the West that it is arming the rebels.
Meanwhile, Russia and Ukraine continued to haggle over a mammoth Russian aid convoy parked near the border as officials said inspections of the roughly 300 lorries would not start Sunday.
An AFP journalist saw 16 trucks drive from a parking lot where they have been idling since Thursday to a Russian border post some 30 kilometres away.
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 Russian and Ukrainian officials agreed on procedures to check the cargo —- supposedly bound for Lugansk — but insisted that "security guarantees" are still needed on how the vehicles could cross rebel-held territory.
"There will be a meeting tomorrow between all the parties involved. There will be no inspection of the lorries today," Paul Picard, a monitor from the Organisation for Security and Cooperation in Europe (OSCE) told AFP at the border.
President Petro Poroshenko told 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.