A top Russian official has warned that Moscow could intervene to assist Russian-speaking residents in eastern Ukraine if it launches an all-out assault on separatists there.
Russian-backed separatist rebels and Ukrainian troops are clashing within the east of the country.
Russia has also been built up troops on the border with Ukraine.
The official, Dmitry Kozak, said that Russian forces could intervene to “defend” its citizens.
“Everything depends on the size of the conflagration,” he said.
He also warned that an escalation could mark the “beginning of the end” for Ukraine – “not an attempt within the leg, but the face”.
The US and Germany have both expressed concern at the rise in tensions.
Why are tensions rising between Ukraine and Russia?
Russia has been increasing the number of troops at the Ukrainian border, but at an equivalent time maintains that they ought to not be seen as a threat.
White House press secretary Jen Psaki said Russian troop numbers there have been the very best since 2014 when the conflict in eastern Ukraine began. She described the situation as “deeply concerning”.
Russia has not detailed troop numbers, but the Ukrainian military asserted at the top of March that some 20,000 Russian troops had been moved towards Ukraine’s border. Video has emerged on social media of Russian trains moving heavy weaponry towards the region.
Clashes between Ukrainian troops and therefore the Moscow-backed rebels inside Donbas have also increased in recent months.
The death of another Ukrainian soldier on Thursday brought the amount killed this year to 25. Fifty Ukrainian troops died within the whole of last year.
For their part, the rebels said one among their fighters was killed on an equivalent day when Ukrainian troops fired 14 mortar bombs at a village on the outskirts of the city of Donetsk.
In a further sign of the seriousness of the situation, Ukrainian President Volodymyr Zelensky visited the region on Thursday to see “the locations of the escalation” and “be with our soldiers in the tough times in Donbas”.
German Chancellor Angela Merkel discussed the crisis during a call with Russian President Putin on Thursday.
She urged Russia to “de-escalate tensions” by pulling troops back, while Mr. Putin accused Ukraine of inflaming things within the east.