Ukraine on Thursday 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.
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 Russian-annexed 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-kilometer-wide (19-mile-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".
"The likelihood of the conflict's escalation remains very high," he said in televised remarks from the western Ukrainian city of Lviv.
Remembering a forgotten war
Poroshenko has issued similar warnings on previous occasions.
But he added on Thursday that a further escalation may force him to introduce "martial law and a mobilization" of reserve troops.
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.
French President Francois Hollande warned on Tuesday against any "escalation" of the conflict after telephone talks with 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".
And Ukraine has even become an issue in the US presidential race, with Republican candidate Donald Trump's campaign chief embroiled in allegations of receiving secret payments from Kiev's Russian-backed leaders prior to the former Soviet republic's 2014 pro-EU revolt.
Analysts remain divided about whether Ukraine is about to enter an even deadlier phase of the war or if Russian President Vladimir Putin is simply adopting a tough posture that could give him the upper hand in any final peace deal.
Russian Foreign Minister Sergei Lavrov said on Monday that Moscow remained committed to a stalled European-brokered truce deal 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".
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.
Poroshenko attended a summit of NATO leaders in Warsaw last month in which the alliance agreed to bolster its eastern flank in order to calm fears of Russia in both Ukraine and among other east European states.