East Ukraine's rebel stronghold Donetsk was pummelled on Sunday by the heaviest shelling in a month.
The Organisation for Security and Co-operation in Europe said it spotted an armoured column of troops without insignia in rebel territory.
A two-month-old ceasefire to end a war that has killed 4,000 people has appeared shakier than ever in recent days, with both sides accusing the other of violating the terms of the peace plan.
Journalists in Donetsk said the shelling sounded more intense than at any time since early October, a period when a playground was struck and at least ten people were killed.
Sunday's strikes appeared to come from territory held by both government and rebel forces.
Ukraine has accused Russia of sending 32 tanks and truckloads of troops into the country's east to support the pro-Russian rebels in recent days.
Moscow has long denied its troops operate in east Ukraine, although many have died there.
OSCE's statement that it spotted an unidentified armoured column in rebel territory helps support Kiev's position that Moscow has been sending in reinforcements to protect separatist enclaves.
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