WASHINGTON: US President Joe Biden spoke at length Sunday with Ukrainian counterpart Volodymyr Zelensky about Russia's military buildup along Ukraine's borders, with the pair committing to pursue "diplomacy and deterrence," the White House said.

"The two leaders agreed on the importance of continuing to pursue diplomacy and deterrence in response to Russia's military build-up on Ukraine's borders," according to a readout of the roughly 50-minute call.

"President Biden reaffirmed the commitment of the United States to Ukraine's sovereignty and territorial integrity," the White House added, referring to the American president's position he has established on multiple occasions when addressing Moscow's brinkmanship.

With Western fears growing that Russia is about to invade the ex-Soviet state, the readout added that Biden "made clear that the United States would respond swiftly and decisively, together with its allies and partners, to any further Russian aggression against Ukraine."

Washington and its allies have warned that Russia has massed well over 100,000 troops on its borders with Ukraine.

Earlier Sunday, top American officials offered grim assessments of the situation, with US national security advisor Jake Sullivan saying a Russian invasion of Ukraine could begin "any day now," and would likely start with "a significant barrage of missiles and bomb attacks" leading to civilian deaths.

Biden on Saturday held an hour-long call with Russian President Vladimir Putin.

But with that conversation failing to move the needle or significantly tamp down sky-high tensions, Pentagon spokesman John Kirby said that was "certainly not a sign that things are moving in the right direction."