Ukrainian President Oleksander Turchynov said on Thursday that 14 troops had been killed in two overnight attacks by rebel forces in the separatist east of the country.
Security sources said on Thursday that the main clash took place about 20km south of the industrial hub of Donetsk, which is now in the hands of separatists who say they will disrupt the election.
Ukrainian forces also fought separatists in the neighbouring Luhansk region but there was no word about any casualties there, the Reuters news agency reported.
The defence ministry confirmed that several people died in a firefight near Donetsk, but gave no precise death toll. It added that the clash occurred when gunmen opened fire on an army checkpoint near the town of Volnovakha.
Separately, Ukrainian border guards said they rebuffed an attempt by dozens of separatists, armed with grenade launchers and rifles, to enter the Luhansk region overnight from Russia. Several guards were hurt in the fighting.
Separatists on Thursday also seized four Ukrainian coal mines in the country\’s east, according to the Ministry of Energy.
"The terrorists, threatening [workers] with guns, are demanding explosives" in the eastern Lugansk region near the Russian border, the ministry said in a statement, adding that the mines belong to the Lysychanskugol company.
Ukrainian security forces and the pro-Moscow separatists have clashed repeatedly in recent weeks in eastern Ukraine, where the breakdown of security has rattled the pro-Western interim government in Kiev.
Kiev has acknowledged that Sunday\’s election cannot be held in parts of the Donetsk and Luhansk regions and has accused Moscow of deliberately seeking to undermine Ukrainian democracy, a charge echoed by the United States and European Union.
Russia denies the legitimacy of the current Kiev government, which took over after mass street protests toppled Moscow-backed president Viktor Yanukovich in February. He fled to Russia and in March Russia seized Ukraine\’s Black Sea peninsula of Crimea.