The Kremlin has said it fears the resumption of full-scale violence in Ukraine and could take action to protect civilians there.
In a statement last week from Dmitry Peskov, the spokesman of Russian President Vladimir Putin, the leader expressed his desire to prevent Ukraine from using force to try to reclaim control over separatist-controlled territory in the country’s east.
Ukrainian forces and Russia-backed separatists have been fighting in the area since shortly after Moscow's 2014 annexation of Ukraine's Crimea Peninsula.
More than 14,000 people have died in the conflict to date and efforts to negotiate a political settlement have stalled, with troops now amassing along the border.
Ukraine and the West have accused Russia of sending in troops and weapons to help separatists, accusations roundly denied by Moscow.
Western and Ukrainian officials have raised concerns in recent weeks about an increasing number of ceasefire violations by Russia in the country's industrial heartland, Donbas.
In a call with Mr Putin last Thursday, German Chancellor Angela Merkel "called for the removal of these troop reinforcements in order to achieve a de-escalation of the situation".
White House Press Secretary Jen Psaki has also raised concerns about the troop build-up in the area, noting that Russia now has more troops along the border than at any time since 2014.
In response, Mr Peskov, the Russian president's spokesman, said Russia is free to deploy their troops wherever they want in its own territory.
In return, he accused the Ukrainian military of an "escalation of provocative actions".
"The Kremlin has fears that a civil war could resume in Ukraine, and if a civil war, a full-scale military action resumes near our borders that would threaten the Russian Federation's security," Mr Peskov added.
"The ongoing escalation of tensions is quite unprecedented."
Dimitry Kozack, a Putin aide who serves as Russia's top negotiator with Kyiv, warned Ukraine that the use of force to retake control of the east would mean "the beginning of an end for Ukraine".
Mr Kozak said Russia would likely protect civilians there if they faced a potential massacre like the one that took place in the 1995 Bosnian War in Srebrenica.
Cover image: Vladimir Putin during a meeting at the Kremlin in 2019 (Picture: PA).