Ukrainian war planes bombarded separatists along a broad front today, inflicting huge losses, Kiev said, after President Petro Poroshenko said "scores and hundreds" would be made to pay for a deadly missile attack on Ukrainian forces.

In exchanges marking a sharp escalation in the three-month conflict, jets struck at the "epicentre" of the battle against rebels near the border with Russia, a military spokesman said.

The planes targeted positions from where separatists, using high-powered Grad missiles, bombarded an army motorised brigade on Friday, killing 23 servicemen.

Warplanes also struck at targets near Donetsk, the east's main town where rebels have dug in, destroying a powerful fighter base near Dzerzhinsk, Andriy Lysenko, a spokesman for the "anti-terrorist operation" said.

"According to preliminary assessment, Ukrainian pilots ... killed about 500 (rebel) fighters and damaged two armoured transporters," Lysenko told journalists.

In an earlier air attack on a base near Perevalsk, north of Donetsk, two tanks, 10 armoured vehicles and "about 500" rebel fighters were destroyed, he said.

Rebel representatives, quoted by Russian news agencies, denied they suffered big losses and said the Ukrainians were using outdated intelligence on where separatist forces were deployed.

"There were no volunteers (rebels) where the Ukrainian aviation was active yesterday," said a spokeswoman for the Luhansk-based separatists, referring to the Peravalsk attack.

Earlier, the border guard service said jet fighters were scrambled to strike at the pro-Russian separatists after they resumed missile attacks on government forces deployed near the frontier with Russia, south-east of the city of Luhansk.

In the military action, which began on Friday evening and continued well into Saturday, five Ukrainian servicemen were killed, Lysenko said. There were 16 overflights by Ukrainian fighter jets in all, he said.

The surge in violence on Ukraine's border with Russia, south east of Luhansk which is controlled by separatists, sparked fresh Ukrainian accusations against Russia of involvement in the border fighting.

Ukraine's Foreign Ministry demanded Russia stop supporting armed groups in its eastern region, and end "provocations" on the border.

"The Russian side cynically disregards the fact that Ukrainian servicemen and peaceful civilians are being killed at the hands of terrorists whom it is supporting," the Foreign Ministry said.

"But, on the other hand Russian border guards complained in a letter about Russian household pets being killed by a supposed Ukrainian artillery shell, 500 metres (yards) from the border."

Rebels had also carried out mortar and missile bombardment of army checkpoints at Dyakove and Nyzhnoderevechka near Luhansk, the "anti-terrorist operation" said.

Journalists based in Donetsk said Ukrainian forces shelled Maryinka, a suburb, on Friday night and apartment blocks bore traces of fire on Saturday.

Igor Strelkov, separatist commander in Donetsk, said his men had headed off a plan to move fighters and armour into the area. Subsequent Ukrainian shelling had killed 30 civilians and the number of casualties could rise, he said.

Vladyslav Seleznyov, main spokesman for the "anti-terrorist operation", said the violence there had been caused by rebels out to discredit the Ukrainian armed forces.

Poroshenko, whose forces recently seemed to be prevailing over the rebels, vowed on Friday to "find and destroy" rebels responsible for the missile attack at Zelenopillya, which also wounded nearly 100 and was one of the deadliest yet against government forces.

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