In a report covering attacks on Friday, its monitors recorded 591 breaches in Donetsk and 975 violations in neighbouring Lugansk, two regions partly held by Russian-backed separatists.

A map breaking down the breaches showed the heaviest fighting now in the northwestern section of the Lugansk region, about 20 kilometres (12 miles) southeast of the government-held city of Severodonetsk.

The shelling in the eight-year conflict spiked sharply this week, as fears mount that Russia is paving the way for an invasion of Ukraine designed to reverse its pro-Western course.

Ukraine's Interior Minister Denys Monastyrskiy experienced the clashes first-hand Saturday, ducking for cover as mortar shells fell within a few hundred metres of him while he toured the frontline with reporters.