US President Joe Biden has used a first official phone call with President Volodymyr Zelenskiy to offer staunch support in Ukraine’s standoff over Russian troops near its border.

Washington has been Ukraine’s most powerful ally since Russia’s annexation of Crimea in 2014 but ties were overshadowed by Ukraine’s unwilling involvement in events leading to the impeachment trial of Biden’s predecessor Donald Trump.

The conversation came after Western nations and transatlantic military alliance NATO voiced concern over Russian troop movements near the eastern Donbass region where Ukrainian troops are in conflict with Russian-backed forces.

“President Biden affirmed the United States’ unwavering support for Ukraine’s sovereignty and territorial integrity in the face of Russia’s ongoing aggression in the Donbass and Crimea,” a US statement said.

The call lasted about 50 minutes, according to Zelenskiy’s chief of staff.

“We discussed the situation in Donbass in detail. President Biden assured me that Ukraine will never be left alone against Russia’s aggression,” the Ukrainian president said in a video statement afterwards.

Russia said this week that an escalation in Donbass could “destroy” Ukraine and warned against any NATO deployment.

Ukraine, Western countries and NATO accuse Russia of sending troops and heavy weapons to prop up proxies in Donbass who seized a swathe of eastern Ukraine in 2014.

Russia says it only provides political and humanitarian support to separatist fighters in what it casts as an internal conflict.

“We stand shoulder to shoulder when it comes to preservation of our democracies,” Zelenskiy wrote on Twitter after the call.