WASHINGTON, Feb 12 - U.S. President Joe Biden warned his Russian counterpart Vladimir Putin that the United States and its allies would "respond decisively" if Russia "further" invaded Ukraine, the White House said on Saturday.

The leaders spoke on a secure call for about an hour on Saturday morning regarding the troop buildup around Ukraine, Reuters reported citing a White House official.

"President Biden reiterated that a further Russian invasion of Ukraine would produce widespread human suffering and diminish Russia's standing," the White House said.

A U.S. official said the call produced no fundamental change and it was unclear if Russia would pursue diplomacy or military action.

While the United States was prepared to engage in diplomacy, Washington remained "equally prepared for other scenarios," the White House added.

Meanwhile, U.S. National security adviser Jake Sullivan on Sunday described how a potential Russian invasion would play out if it were to happen.

"If there is a military invasion of Ukraine by Russia, it's likely to begin with a significant barrage of missiles and bomb attacks," Sullivan said. "Those are never as precise as any army would like them to be, and we don't even know how precise the Russian army would like them to be, so innocent civilians could be killed regardless of their nationality."

During an interview on CNN's "State of the Union," Sullivan continued by predicting there would be an "onslaught of a ground force moving across the Ukrainian frontier" following the bomb attacks if they occur, adding that civilians could get caught in the crossfire if this invasion takes place.

"We will defend NATO's territory," Sullivan said. "We will impose costs on Russia and we will ensure that we emerge from this as the West stronger, more determined, more purposeful than we have been in 30 years, and that Russia ultimately suffers a significant strategic cost for military action."

Sullivan said the Biden administration could not "perfectly predict" which day Russia would invade Ukraine, but said it could begin "any day now," stating that the way Russia has "built up their forces" and "the way they have maneuvered things in place makes it a distinct possibility that there will be major military action very soon."

Sullivan also reiterated a warning from the Biden administration to American citizens in Ukraine, telling them to leave Ukraine "immediately" and that there will not be a military evacuation. The State Department on Saturday said Americans should escape through Poland. (rt/ez)