KYIV, Aug. 6 - Four Ukrainian soldiers have lost their lives in the latest clashes with Russia-backed separatists in the country's east, two weeks after a cease-fire has been declared to allow grain harvesting in the region.

The Defense Ministry said in a statement that the pro-Russia separatists opened fire at Ukrainian military positions in the Donetsk region on Tuesday, using grenade-launchers, machine guns, and assault rifles, RFE/RL reported.

"We say with sadness that, as a result of the enemy's attacks today, according to the information in our possession, four of our heroes sustained injuries, to which they succumbed," the statement said.

It was the highest daily casualty toll in the Ukrainian conflict since a truce was agreed nearly three weeks ago.

President Volodymyr Zelenskiy called the incident an attempt to "undermine" Ukraine's efforts toward peace, and called on France, Germany, and Russia to "meet as quickly as possible to resume negotiations."

Germany, France, Ukraine, and Russia are part of the so-called Normandy format for talks aimed at putting an end to fighting between Ukrainian forces and the separatists who control parts of the Donetsk and Luhansk regions.

Zelenskiy wrote on his Facebook page that Kyiv "will not cease to work toward ending hostilities or leave any attack on our soldiers unanswered."

Since April 2014, more than 13,000 people have been killed in the conflict.

A new cease-fire agreement was supposed to go into effect on July 22 in accordance with an agreement reached in Minsk on July 17 by Ukrainian and Russian envoys as well as members of the Organization for Security and Cooperation in Europe (OSCE).

But the sides have been accusing each other of violating the agreement since then.

Cease-fire road maps announced as part of the Minsk accords -- September 2014 and February 2015 pacts aimed at resolving the conflict -- have contributed to a decrease in fighting but have failed to hold. (rfe/ez)