Ukraine on Thursday, August 18 reported the heaviest rebel shelling attack in the separatist east for a year in what the president said could be a prelude to a full-scale Russian invasion, AFP reports.

Military spokesman Oleksandr Motuzyanyk said insurgent strikes had doubled from the previous day as tensions between Kiev and Moscow soar over Kremlin charges that Ukraine plotted to make armed incursions into Crimea this month.

"The rebels launched more than 500 mortar and over 300 artillery shells at our positions," Motuzyanyk told reporters in Kiev.

"The last time we witnessed a similar intensity of fire using heavy armaments was a year ago."

Motuzyanyk said three Ukrainian soldiers were killed and six wounded in clashes across the 30-kilometre-wide buffer zone splitting government forces from the pro-Russian militias.

Kiev and its Western allies accuse Moscow of trying to escalate a 28-month conflict in Ukraine's rust belt that has claimed more than 9,500 lives and began just weeks after Russia annexed the Black Sea peninsula of Crimea in March 2014.

Petro Poroshenko responded to the reported violence by sending his top army commander into the war zone and warning that he "does not exclude a full-scale Russian invasion along all fronts," AFP says.

"The likelihood of the conflict's escalation remains very high," he said in televised remarks from the western Ukrainian city of Lviv.

The Crimea episode has thrust back into the spotlight a conflict that has effectively ground to a stalemate but remains one of Europe's bloodiest since the 1990s Balkans wars.

Kiev and the West accuse Russia of supporting the rebels and deploying troops across the border -- claims Moscow denies -- in order to keep at least a part of its western neighbor within its geopolitical orbit.