KIEV - Escalating clashes between pro-Kremlin separatists and Ukrainian forces on Sunday killed 18 civilians and forced the new Western-backed leader to cancel a pivotal meeting with Russian President Vladimir Putin at the World Cup in Brazil.

An explosive security crisis just outside the eastern border of the European Union that has claimed more than 550 lives and enflamed East-West ties threatened to spiral into an all-out civil war over the weekend.

Militias that the West and Kiev allege are being armed by the Kremlin used a Grad multiple-rocket system late Friday to mow down 19 Ukrainian soldiers and wound nearly 100 near the Russian border.

Further attacks killed 18 more troops and 26 civilians - 18 of them in what Kiev said were missile and other overnight rebel strikes staged across the eastern rustbelt - in violence that appeared to shatter any hope of a truce.

Kiev-backed authorities said 12 people were killed and at least eight wounded in a suburb of the almost million-strong rebel stronghold of Donetsk.

Municipal workers in neighbouring Lugansk said six people had also died and seven were injured in various overnight incidents in the other separatist bastion of 425,000.

Separatist commander Igor Strelkov said Ukrainian forces responded to the latest surge in violence by sending several dozen tanks to the outskirts of Lugansk in preparation for a possible invasion of the Russia border city.

Russia’s RT state television showed several tanks moving along paved city streets of what the channel said Sunday were in Lugansk.

Ukrainian defence officials issued no immediate comment and the Russian state TV footage could not be independently verified.

The civilian toll is one of the highest recorded over a two-day span in a three-month conflict that has threatened the very survival of the strategic ex-Soviet state.

And the military losses have profoundly dampened emerging hopes in Kiev that its recent string of battlefield successes had finally convinced the rebels to sue for peace.

Ukrainian President Petro Poroshenko has vowed to kill “hundreds” of gunmen for every lost soldier and ordered an air-tight military blockade of Lugansk and Donetsk - both self-proclaimed capitals of their own “People’s Republics” that want to join Russia.

European leaders responded by joining forces with Putin in a bid to convince Poroshenko to put the breaks on violence first sparked by the February ouster of a Kremlin-backed leader and fanned by Russia’s subsequent seizure of Crimea.

The immediate hopes of a truce rested on a meeting between Putin and Poroshenko - the second since the Ukrainian president’s May 25 ballot - that seemed in the cards on the sidelines of the World Cup final in Rio de Janeiro.

Putin was due in Rio as part of a Latin American tour and the Brazilian presidency said Poroshenko had also accepted an invitation on Friday.

But the Ukrainian leader’s office said early on Sunday that Poroshenko was forced to cancel his attendance “considering the situation currently happening in Ukraine”.

The separatists’ use of Grad systems - featured heavily in Russia’s devastating assault on the Chechen capital Grozny in the 1990s - has underpinned the most recent charges that the Kremlin is directly involved in the insurgency.

Putin rejects accusations of orchestrating the uprising to retain partial control over eastern Ukraine and punish Kiev for its decision to strike an historic EU alliance instead of a new Kremlin pact.

But Poroshenko argues that no truce with the rebels is possibile until his troops manage to seal the Russian border and halt the continuing flow of gunmen and arms.

The frontier became the conflict’s new frontline after last weekend’s evacuation by the rebels of a host of towns and cities that they had held since early April in the coal mining region of Donetsk.

The militias have since concentrated their forces around the cities of Donetsk and Lugansk and are hoping for new weapons deliveries to revive their campaign.

Fresh tension emerged with claims by Moscow of a Ukrainian shell killing a civilian near a small Russian border town.

The Russian foreign ministry called the episode “another act of aggression” that could have “irreversible consequences” for Ukraine. A spokesman for Ukraine’s national security and defence council denied Kiev’s involvement.