Ukrainian President Petro Poroshenko vowed to "find and destroy" pro-Russian rebels who killed 23 servicemen and wounded nearly 100 in a missile attack on Friday. Earlier reports had put the death toll at 30, which was later revised down.
Poroshenko issued his angry statement following an emergency meeting of his security chiefs called in response to the early morning strike by Russian-made Grad missiles on an army motorized brigade near the border with Russia.
"For every soldier's life, the militants will pay with scores and hundreds of their own," Poroshenko wrote on his website. "All those who used the Grad against the Armed Forces of Ukraine will be found and destroyed."
If the death toll is confirmed, it would amount to the deadliest rebel attack on government forces since the Ukrainian military ended a unilateral cease-fire on June 30. The deadly incident came in retaliation against the attack that pushed the rebels out of a stronghold in Slovyansk last week.
The pro-Russian separatists launched their attack at around 5 a.m. on the border post at Zelenopillya, in Luhansk region, military sources said.
Government forces have recently been gaining the upper hand in a three-month battle with separatists who have set up "people's republics" in the Russian-speaking east of the country and said they want to join Russia.
Vladyslav Seleznyov, a Ukrainian military spokesman, said at least 50 separatist fighters had been killed in the past few days following air attacks against rebel positions near the border with Russia.
Last weekend, after Poroshenko refused to prolong a 10-day unilateral cease-fire when it expired on June 30, he scored a notable victory by pushing rebels out of a stronghold in Slovyansk and forcing them back to the main industrial city of Donetsk where they have now dug in.
Rebels in Donetsk, commanded by a Russian called Igor Girkin, or Strelkov, say they are recruiting new fighters to resist government forces despite a lack of help from Russia, which gradually withdrew troops from its border.
In mid-June, in Luhansk region, rebels downed a plane, killing the 49 Ukrainian servicemen on board. Buoyed by the success in Slovyansk, Ukrainian security officials say they have a plan ready to crush the rebels in Donetsk, though Poroshenko has ruled out airstrikes and artillery bombardment because of the large civilian population. He also ruled out any more cease-fires such as the one in June until the rebels lay down their arms.
French President Francois Hollande and German Chancellor Angela Merkel urged Russian President Vladimir Putin by telephone Thursday to exert pressure on the separatists to de-escalate the violence.
Poroshenko told U.S. Vice President Joe Biden that Russia and the separatists had refused multiple proposals by Kiev for venues to negotiate a cease-fire, the White House said.
Al Jazeera and wire services