Separatist gunmen attacked a Ukrainian military base with Russian-made Grad rockets, killing 19 troops and boosting the overnight death toll to about 30 in the latest flare-up of fighting in eastern Ukraine, Russian media and Ukrainian officials said Friday.
It was the deadliest day for the Kiev government since fighting resumed last week after a 10-day cease-fire and damped a mood of resurgency that followed the Ukrainian troops' success last weekend in sweeping out militants from their strongholds in Slovyansk and Kramatorsk.
"The high number of casualties was caused by a sudden missile attack from the terrorist positions close to Russia's border," Alexei Dmitrashkovsky, spokesman for the government mission to retake occupied territory from the separatists, said in an interview at the operation's base in Izyum, just north of Slovyansk.
The sophisticated Grad systems are smuggled in from Russia across border checkpoints under militants' control, Dmitrashkovsky said, adding that Ukrainian forces have intercepted and destroyed five of the multiple-launch systems in recent weeks.
Ukrainian President Petro Poroshenko vowed to avenge the rocket attack on the military base at Zelenopillya, on the Russian border in the Luhansk region.
"For every life of our soldiers the rebels will pay with tens and hundreds of their own," Poroshenko said in a statement on the presidential website.
Four other Ukrainian troops were killed in a separate attack on border guards near Luhansk city, military spokesman Vladyslav Seleznev said in a post on his Facebook page, which put the overnight death toll at 23 government forces.
Separatists armed with Russian-made guns and commanded by a Russian special forces veteran also ambushed a bus carrying miners home from the DTEK enterprise near Luhansk, killing five of the workers, Interfax news agency reported.
DTEK is part of Ukrainian magnate Rinat Akhmetov's sprawling industrial empire, which has been targeted by the rebels.
Interfax quoted the director of Akhmetov's eastern enterprises as saying the bus attack had forced the company to suspend operations at four mines in the Luhansk region that employ 4,500.
Russia's RIA Novosti news agency quoted a representative of the militants' proclaimed Peoples Republic of Donetsk as saying the attack in Zelenopillya was believed to have killed at least 30 Ukrainian troops and destroyed an armored convoy of the 24th Motorized Division from Lviv.
The overnight attacks showed that the separatists remain capable of inflicting serious harm on the government as it carries out what it calls an "anti-terrorist operation" to recover control of administrative buildings, police stations and border crossings.
Ukrainian authorities and their Western allies accuse Russian President Vladimir Putin of arming and instigating the rebellion after Russian troops seized and annexed Ukraine's Crimean peninsula in March.
Putin has denied having a hand in the violence that has taken well over 500 lives since late March. However, dozens of separatist fighters killed in the clashes over the last three months have been identified as Russian citizens and some, like Donetsk military commander Igor Strelkov, openly concede they came from Russia to aid the insurgency.
Williams reported from Los Angeles and Loiko from Slovyansk.