Tensions between Russia and Ukraine have been at a fever pitch since the Crimea crisis in 2014. The U.S. and its European allies have voiced concerns over Russia's military buildup along Ukraine's border. 

Moscow said there was no aggressive intention there, and it needs to ensure its security in response to alleged threats from the North Atlantic Treaty Organization (NATO), a Western defense alliance. 

Russia's UN ambassador Vassily Nebenzia said Monday that his country has no plans to invade Ukraine and accused the West of whipping up tensions and provoking escalation.

It is not the first time an apparent buildup of Russian arms on the Ukraine border has caused consternation in Europe and NATO. In the spring of 2021, fears escalated as a result of troop movements on the Russian side of the border.

Ukraine's Foreign Minister Dmytro Kuleba said the latest movements were a continuation of the earlier pressure his country had faced.

"In Ukraine's view, what we currently see along the border is not a purely military buildup because Russia already brought a military armada to our borders in spring and has never really withdrawn it since," he said. 

"What we see now is a deteriorating situation where Russia is demonstrating that it can quickly activate the already amassed troops and equipment and that every option, including the military one, is on the table for the Russian leadership."

Here is a close look at Russia-Ukraine ties since 1991.

What motivated Russia's moves against Ukraine?

Scholars are at odds about the motivations behind Russia's move toward Ukraine. Some believe that Russia has viewed NATO's post–Cold War enlargement with increasing alarm. 

In 2004, NATO added seven members, its fifth expansion and the largest one to date, including the former Soviet republics Estonia, Latvia and Lithuania. 

Four years later, when NATO declared its intent to bring Ukraine and Georgia into the fold at some point in the future, Russia made clear a red line had been crossed.

Live update: Russian ambassador says no plans to invade Ukraine