The joint military command for east Ukraine reported 66 exchanges of fire by 7:00 am (0400 GMT) -- a high number compared to recent stages of the conflict -- while the rebels called the situation "critical".

The statement said the rebels had resorted to using banned 82 and 120 millimetre-calibre mortar shells in towns across the front running in the eastern regions of Lugansk and Donetsk.

"There have been no losses among members of the armed forces as a result of the enemy's attacks," it said.

"The armed forces control the situation and continue to perform their mission to rebuff and contain the armed aggression of the Russian Federation."

Moscow formally denies being involved in the conflict and calls it Ukrainian internal affairs.

But monitors from the OSCE European security body have reported regular shipments of Russian weapons across the border throughout the eight-year war.

The OSCE reported a massive 870 ceasefire violations across the conflict zone in its latest report Friday, which referred to incidents on Thursday.

"In recent days, the OSCE Special Monitoring to Ukraine (SMM) has observed a dramatic increase in kinetic activity along the contact line in eastern Ukraine," the OSCE said in a statement.

Rebel leaders accuse the Ukrainian armed forces of trying to retake their two separatist regions by force -- a claim Kyiv denies.

The rebel leadership in Donetsk on Saturday called the situation "critical" and announced a "general mobilisation".

Th leader in the smaller separatist Lugansk region did the same about an hour later.