A man and his dog walk together July 11 in the village of Semyonovka, near the eastern Ukrainian city of Slovyansk. (Alvaro Ybarra Zavala/Getty Images)

Ukrainian President Petro Poroshenko called for revenge after 19 Ukrainian soldiers were killed in a rocket attack Friday, one of the deadliest days for security forces since the beginning of the country’s months-long conflict.

The attack, in which almost 100 soldiers were wounded, was a bloody reminder that Ukraine’s rebels retain the ability to inflict massive damage despite territorial setbacks in the past week. Four border guards were also killed in a separate incident, and intense fighting continued throughout Ukraine’s restive east.

“Militants will pay hundreds of their lives for each life of our servicemen,” Poroshenko told his top security officials Friday. “Not a single terrorist will avoid responsibility. Each of them will be punished.”

The soldiers were killed in a rocket attack near the town of Zelenopolye, about three miles from the Russian border in Ukraine’s southeastern Luhansk region, Zoryan Shkiryak, an adviser to Ukrainian Interior Minister Arsen Avakov, said Friday at a briefing in Kiev. A military spokesman later said that the day’s death toll was 23, the highest one-day figure since separatists shot down a Ukrainian military plane last month, killing 49.

The Ukrainian military has been trying to seal the border with Russia for weeks after rebels seized several key border posts. Ukrainian officials have said that a sealed border is the only way to ensure that Russian equipment and volunteers do not flow over it.

Poroshenko said the deaths had been caused by a Russian-made Grad multiple rocket launcher, a heavy artillery vehicle that rains down rockets on its targets.

Rebels have made major retreats in the past week, and the Ukrainian government says rebel-held territory has shrunk by half. But separatists have fortified themselves in the city of Donetsk, which has 1 million residents, and they have been defiant about their intent to remain and not give up their fight. Ukrainian military officials, meanwhile, have said that they will surround the city and force the rebels to surrender.

A top rebel leader, Igor Girkin, also known as Igor Strelkov, said Thursday that the rebels were planning to start paying their fighters, an apparent attempt to boost their quality and commitment.

Also Friday, the European Union announced that it would impose new travel bans and asset freezes on 11 individuals that it said had been contributing to the fighting in eastern Ukraine. The bloc of 28 nations said in a statement that the names would be announced Saturday.