Observers from the OSCE European security body on Saturday reported more than 1,500 ceasefire violations in east Ukraine in a single day, the highest number this year.
In a report covering attacks on Friday, its monitors recorded 591 breaches in Donetsk and 975 violations in neighboring Luhansk, 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 Luhansk region, about 20 kilometers (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 meters of him while he toured the frontline with reporters.