People stand by a bus in the Rostov-on-Don region, Russia, early on Saturday, after evacuating from Donetsk. AP

The Ukrainian military said it had recorded 12 ceasefire violations by pro-Russian separatists in east Ukraine on Saturday morning after 66 cases over the previous 24 hours.

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Earlier, the leader of the east Ukraine breakaway region, the so-called Donetsk People's Republic, on Saturday said he had introduced general mobilisation, with fears mounting of war in the ex-Soviet country.

Separatists opened fire on more than 20 settlements, using heavy artillery, which have been prohibited by Minsk agreements, the military said on its Facebook page.

People walk from a bus after evacuating from Donetsk near the border with Ukraine, Russia, on Saturday. AP

Incidents of shelling across the line dividing government forces and separatists increased sharply this week, in what the Ukrainian government called a provocation. It strongly denied suggestions by Russia that Kyiv could launch an offensive in eastern Ukraine.

"I urge my fellow citizens who are in the reserves to come to military conscription offices. Today I signed a decree on general mobilisation," Denis Pushilin said in a video statement, after monitors warned of large uptick in fighting in east Ukraine.

The separatist conflict in eastern Ukraine erupted in 2014 and has killed over 14,000 people.