Escalating clashes between pro-Kremlin separatists and Ukrainian forces have 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 relations 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 on 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 incidents in the other separatist bastion of 425,000.

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

Russia’s RT state television on Sunday showed two tanks moving along paved city streets of what the channel said 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.