Ukrainian President Poroshenko said Kyiv might order a new wave of military mobilization and institute martial law in response to more violence in the east. Both sides report heavy shelling, despite the shaky truce.
At least three Ukrainian soldiers were killed and six more wounded in the last 24 hours, military spokesman Oleksandr Motuzyanyk said on Thursday, marking the deadliest clashes with pro-Russian rebels in recent months.
"The rebels launched more than 500 mortar and over 300 artillery shells at our positions," Motuzyanyk told reporters in Kyiv.
"The last time we witnessed a similar intensity of fire using heavy armaments was a year ago," he added.
According to Motuzyanyk, the soldiers were killed across the 30-kilometer (19-mile) buffer zone separating the two sides' forces.
Donetsk-based rebels also reported their positions were shelled on Wednesday night.
The spike in fighting threatens to unravel a truce Russia and Ukraine agreed on in February 2015. In addition to latest deaths, the Organization for Security and Co-Operation in Europe cited a "high number" of truce violations this week, including artillery, gunfire and the movement of heavy weapons.
On Thursday, Ukrainian President Petro Poroshenko said the government might respond by instituting martial law.
"If the situation escalates in the east and in Crimea, we don't rule out the possibilities [that] we will be forced to introduce martial law and announce a [further] mobilization," he said.
Kyiv has already ordered several waves of mobilization since the conflict started in 2014.
'Everybody' interested in peace
High-ranking rebel commander Eduard Basurin said his forces had no plans of mobilizing civilians in the self-proclaimed Donetsk republic.
"Our army is formed solely on the principle of volunteering," he said.
"Our stance is a defensive one. Everybody is interested in resolving the conflict peacefully," he told the Russian news agency Interfax. "However, if the opponent moves forward, we would be forced to provide an adequate response."
New tensions between Kyiv and Moscow have soared recently as Russia accused Ukraine of a plot to secretly enter Crimea and commit terror strikes. Ukrainian officials denied the charges.
The fighting in Ukraine has so far claimed over 9,500 lives.