Ukraine has said that pro-Russian rebels had killed three of its soldiers in the worst violence seen in the separatist east for a year.

Oleksandr Motuzyanyk, the military spokesman, said on Thursday the rebel attacks had doubled from the previous day as tensions between Kiev and Moscow soared over Kremlin charges that Ukraine plotted to make armed incursions into Russian-annexed Crimea this month.

He said that six more Ukrainian soldiers were also wounded in the clashes across the 30km-wide buffer zone separating the two sides' forces.

"The rebels launched more than 500 mortar and over 300 artillery shells at our positions," Motuzyanyk said in Kiev. "The last time we witnessed a similar intensity of fire using heavy armaments was a year ago."

READ MORE: Ukraine divided - stories from warring sides

Ukraine and its Western allies say that Moscow is simply trying to escalate a 28-month separatist conflict in the country's east that has claimed more than 9,500 lives and began just weeks after Russia annexed the Black Sea peninsula in March 2014.

French President Francois Hollande warned on Tuesday against any "escalation" of the conflict after telephone talks with his Ukrainian counterpart Petro Poroshenko and German Chancellor Angela Merkel.

EU President Donald Tusk on Wednesday said he and the Ukrainian leader both believed Russia's account of recent events in the battle-scarred east and Crimea was "unreliable".

Russian Foreign Minister Sergey Lavrov said on Monday that Moscow remained committed to a stalled European-brokered peace plan that was signed in the Belarussian capital Minsk in 2015.

But he also warned that Russia would take "comprehensive measures to make sure any attempts to make incursions into our territory are nipped in the bud".

Source: Agencies