Pro-Russian rebels killed at least nine Ukrainian soldiers in the east overnight, the defence ministry said Thursday, the bloodiest day for the military since the start of the separatist uprising. The attacks dealt a severe blow to government forces just three days before Sunday's presidential election aimed at leading the country out of its deep crisis. In the deadliest strike, eight soldiers were killed and another 17 wounded when rebels opened fire on a military checkpoint near the town of Volnovaha in the Donetsk region, ministry spokesman Bogdan Senk told AFP. The ministry said the militants fired mortar shells and grenades at the barricade, blowing up a military vehicle. Another soldier was killed and two injured in an attack near Rubizhne in the neighbouring Lugansk region, it said. The military did not say if there were any dead or injured among the separatists in the two attacks. Kiev's interim government launched what it describes as an "anti-terrorist" operation in mid-April aimed at crushing the rebels who have taken control of over a dozen towns in the eastern industrialised region. But it has lost a number of men since then, including seven in an ambush between the eastern flashpoint of Slavyansk and Kramatorsk on May 13. An estimated 140 people have been killed in combat since mid-April, according to an AFP tally based on UN and Ukrainian defence ministry figures.