Sunday's phone call came ahead of de-esclataion talks in Geneva between senior US and Russian officials.
US President Joe Biden has told Volodymyr Zelenskyy that the West will act "decisively" if Russia further invades Ukraine.
Biden further pledged to support Kyiv amid a buildup of Russian troops near Ukraine's eastern border over the past several weeks.
The phone call on Sunday also came ahead of a series of diplomatic meetings in Geneva between senior US and Russian officials to try to de-escalate the crisis.
“President Biden made clear that the United States and its allies and partners will respond decisively if Russia further invades Ukraine,” White House press secretary Jen Psaki said in a later statement.
In a tweet on Sunday, Zelenskyy added that Ukraine appreciated the "unwavering support".
"We discussed joint actions by Ukraine, the United States, and its partners in keeping peace in Europe, and preventing further escalation," he added.
The US has previously mulled hitting Russia with economy-jarring sanctions if it moves on Ukraine’s territory, but Biden said last month that US military action was off the table.
The Kremlin has demanded that any further expansion of NATO should exclude Ukraine and other former Soviet countries. Moscow has also demanded that the military alliance remove offensive weaponry from countries in the region.
The upcoming talks in Geneva will also be followed by meetings at the NATO-Russia Council, and at the Organization for Security and Cooperation in Europe.