Image copyright PA Image caption Ukraine’s military is fighting pro-Russian separatists in eastern Ukraine
The Ukrainian military says Russian troops have crossed the border into Ukraine twice in the past three days.
A Ukrainian military spokesman told the BBC there were “120,000 soldiers” on Ukrainian territory – which was the highest estimate of Russian troops since 2014.
He said the Russian army was expanding its zone of control, bringing the country closer to the town of Debaltseve.
The Russian defence ministry said the “military action by Ukrainian forces against the occupied regions is an attempt to provoke Russia”.
“Ukraine has made no attempt to show up in Debaltseve, this is the exact opposite,” spokesman Igor Konashenkov said in a statement.
A military spokesman from Ukraine’s National Guard told the BBC this was the first time that Russian forces had intervened in the conflict at this scale.
“In the days of Soviet times, we had 300,000 troops in our western borders and there were no Russians here,” Stepan Poltorak said.
In 2014, the Russian-backed separatist rebellion in Ukraine broke out, and Russia was accused of providing arms, troops and training to its proxies.
Kiev launched an operation to reclaim control of eastern Ukraine, and Russia banned Ukraine from using its airspace.
Since the conflict broke out, more than 10,000 people have been killed.
Mr Poltorak said he had seen the influx of Russian troops on the Ukrainian border over the last few days in the village of Tagus, near Luhansk.
“We can see they have large tanks and armoured personnel carriers on our border, bringing Russian troops to the border which is a provocation, a provocation to our sovereignty,” he said.
Debaltseve has been a major battleground in the conflict, with Ukrainian forces trying to flush out pro-Russian separatists who have been barricaded in the town’s central square since March.
Image copyright Reuters Image caption A 20-year-old soldier was killed by shrapnel as he helped evacuate wounded in Debaltseve
On Friday, Ukraine announced it had recaptured Debaltseve, a week after it pulled out its forces to allow for a ceasefire deal to take effect.
Mr Poltorak said the “taking back of Debaltseve is a turning point for the conflict in the east”.