Germany warned Sunday that direct military clashes between Russia and Ukraine must be avoided at all costs as Kiev and Moscow's top diplomats joined crisis talks with only faint hope of a breakthrough.
A picture shows a Ukrainian multiple rocket launcher "Grad" set on the position near the eastern city of Seversk on July 12, 2014, a day after the town was freed from pro-Russian militants
The foreign ministers of Russia, Ukraine, Germany and France gathered in Berlin after Moscow again denied claims it had sent weapons across the border to prop up the flagging separatist rebellion.
German Foreign Minister Frank-Walter Steinmeier said ahead of the talks that all sides must be "very careful ... about sliding ever deeper into a direct confrontation between Ukrainian and Russian armed forces".
"This must be avoided at all cost," said Steinmeier before meeting Ukraine's Foreign Minister Pavlo Klimkin, Russia's Sergei Lavrov and Laurent Fabius of France.
He cautioned that "there are no guarantees that today’s talks will lead to the success we desire", but stressed that "it would be irresponsible ... not to take advantage of this willingness to talk".
The question would be "whether there is a path toward a ceasefire that is realistic and resilient and that offers us hope of ending the violence," he said.
"We are far from an end to tensions -- people are still dying, no ceasefire has been reached and we are far off a political solution," he added before starting the talks, which were continuing at a government lakeside retreat some three hours later at 0800 GMT.
- Brink of catastrophe -
On the ground in east Ukraine, 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.
Ukraine also ramped up the stakes before the talks in Germany by alleging another military convoy that included 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 which breached the frontier Thursday.
Ukraine's Klimkin said Sunday's talks with his Russian counterpart 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.
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.
- Aid checks to start? -
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 (20 miles) 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 (Monday) 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.
Ukrainian 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.
- Warplane shot down -
Around the region fierce clashes continued between government forces and rebels, with Ukraine's military saying a MiG fighter jet was shot down not far from Lugansk and the pilot parachuted to safety.
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.
Local authorities said that the fighting has cut off water supply and damaged the railroad making it impossible for trains to reach the city.
There were also reports of fierce fighting in the key transport hub Yasynuvata, some 20 kilometres north of Donetsk, following claims from Poroshenko that the town had been captured.