Biden reassures Ukraine's Zelenskyy over Russia buildup

US President Joe Biden has reassured his Ukrainian counterpart Volodymyr Zelenskyy that Washington "will respond decisively" if Russia moves to invade its pro-Western neighbour, the White House said in a statement.

With a Russian military buildup on Ukraine's borders, Biden "made clear" to Zelenskyy during a phone call that the "United States and its allies and partners will respond decisively if Russia further invades Ukraine," White House press secretary Jen Psaki said.

The show of US support for Ukraine comes days after Biden warned Russian President Vladimir Putin of severe consequences if Moscow launches a military invasion.

In his call with Zelenskyy, Biden also stressed Washington's commitment "to the principle of 'nothing about you without you,'" in an apparent reference to the need to include Ukraine in negotiations about its own future.

READ MORE: Biden, Putin hold call to ease tense Ukraine standoff

Zelenskyy appreciates 'unwavering support'

Zelenskyy tweeted afterwards that he appreciated the United States' "unwavering support" and that the call "proves the special nature" of the two countries' relationship.

"Joint actions of Ukraine, the US and partners in keeping peace in Europe, preventing further escalation ... were discussed," he said.

High-ranking US and Russian officials are due to sit down on January 9 and 10 in Geneva to discuss the crisis.

Earlier this week, Biden spoke to Putin, the two leaders' second conversation by phone in three weeks as tensions mount over Ukraine.

Discussing the call with Putin on Friday, Biden said: "I'm not going to negotiate here in public, but we made it clear he cannot – I'll emphasise, cannot – invade Ukraine."

The US leader added, in remarks to reporters during a holiday stay in Delaware, that he had "made it clear to President Putin that we will have severe sanctions, we will increase our presence in Europe, with NATO allies" if Russia invades Ukraine.

Psaki also said in Sunday's post-call statement that Biden reaffirmed "the United States' commitment to Ukraine’s sovereignty and territorial integrity."

The US leader additionally "expressed support for confidence-building measures to de-escalate tensions in Donbass and active diplomacy to advance the implementation of the Minsk Agreements."

READ MORE: Donbass isn’t the guaranteed ally Russia had expected

Tense Ukraine border

Under the Minsk agreement – brokered by France and Germany – Ukraine agreed to carry out political reforms while Russia agreed to end its support for pro-Russian separatist rebels.

Washington and its European allies accuse Russia of threatening Ukraine with a new invasion.

Some 100,000 Russian troops are massed near the border of the country, where Putin already seized the Crimea region in 2014 and is accused of fomenting a pro-Russian separatist war which erupted that same year in the east.

Moscow denies Western allegations and describes the troop presence as protection against the expansion of NATO, although Ukraine has not been offered membership in the military alliance.

READ MORE: Russia warns West of armed response over Ukraine threats