Ukraine has accused Russia of sending dozens of tanks, heavy weapons and truckloads of armed men into the country's rebel-controlled eastern areas.
Andriy Lysenko, a military spokesman, said on Friday that at least 32 tanks and 16 artillery systems, as well as 30 trucks loaded with armed men and ammunition, had crossed into eastern Ukraine from Russia on Thursday.
Lysenko said that another convoy with three mobile radar stations also entered the same area.
He also said that five Ukrainian soldiers had been killed and 16 injured in the past 24 hours in clashes between government forces and rebel fighters that underscored the doubts over a two-month ceasefire that both continue to insist they are respecting.
Ukraine and the West have continuously accused Moscow of fuelling a pro-Russian rebellion in the east with troops and weapons. The accusations have been repeatedly denied by Russia.
Russia made no immediate comment on Lysenko's statement, but earlier it again rejected Western allegations Moscow was deploying more troops near the border. There was no way to confirm the Ukrainian claims independently.
Fifteen civilians were wounded by shrapnel in the separatist bastion of Donetsk, the mayor's office said, in a night of shelling in two neighbourhoods near the ruins of the airport, where government troops are holding out.
Kiev has recently ordered the military to increase the number troops on the frontline to prevent further losses of territory to the rebel.
While the September truce agreement has seen full-scale confrontations halt along most of the frontline, shelling has continued at flashpoints around the industrial east.
Claims of fresh troop movements are reinforcing fears of a return to all-out fighting.
The latest casualties came after Kiev moved to isolate the Kremlin-backed separatist regions, firming up the split of the former Soviet republic in a crisis that has sent East-West relations to their lowest ebb since the Cold War.
The AFP news agency reported that shelling in the fog-shrouded city of Donetsk had subsided on Friday morning.
Al Jazeera's Rory Challands, reporting from Moscow, said there are many people saying that all these things point to crumbling of the peace process,
The rebels held elections on Sunday, defying Kiev with a move that sought to formalise their control over the separatist-held territory.
In response, Ukraine's border guards announced obligatory passport controls around the rebel-held areas on Thursday, effectively setting up a de facto border despite Kiev's insistence that it has not given up on reclaiming sovereignty.
Source: Al Jazeera and agencies