The pro-Russian separatist leader in eastern Ukraine's rebel hub of Donetsk vowed Monday to mobilize 100,000 fighters for their latest offensive against Kiev forces.

"There will be general mobilization in the (separatist) Donetsk People's Republic in 10 days' time, we plan on mobilizing up to 100,000 men," Alexander Zakharchenko told the separatist news agency DAN.

The declaration came after the separatists vowed to push their offensive to take over the entirety of the Donetsk and Lugansk regions and the latest bid to establish a ceasefire collapsed.

Fighting has since intensified, with Kiev officials and local authorities saying Monday that five Ukrainian soldiers and three civilians had been killed in clashes in the last 24 hours.

"Five soldiers were killed and 29 wounded in fighting," military spokesman Vladyslav Seleznyov told journalists in Kiev, adding that Ukrainian forces had been targeted with artillery or mortar fire 104 times.

Two civilians, both men, were killed in a rebel Grad rocket strike in the village of Krimske, around 40 kilometers (25 miles) northwest of separatist center Lugansk, regional pro-Kiev governor Gennadiy Moskal said.

Another man was killed in firing overnight in the other main rebel center Donetsk, city authorities said.

An AFP journalist in the city said firing in Donetsk continued into the night, but was less intensive Monday morning.

Fighting in east Ukraine claimed around 50 lives over the weekend, with the latest attempt at truce talks collapsing in Minsk on Saturday.

More than 5,100 people have been killed in the nine-month conflict, according to the United Nations.