President Joe Biden underscored U.S. support for Ukraine on Friday in his first call to that country's leader following reports of Russian troop movements on its eastern border.

Biden in his conversation with Ukrainian President Volodymyr Zelenskiy affirmed "unwavering support for Ukraine's sovereignty and territorial integrity in the face of Russia's ongoing aggression," according to a statement from the White House.

The United States and Ukraine say Russia is escalating tensions in eastern Ukraine, where Russia-backed separatists have been fighting Ukrainian forces since 2014. Russia denies having a military presence there.

The Pentagon this week accused Russia of violating a 2020 ceasefire, including in a March 26 clash that killed four Ukrainian soldiers. Ukraine's military blamed a Russian mortar attack.

Zelenskiy said in a tweet after the call that Ukraine and the United States "stand shoulder to shoulder when it comes to preservation of our democracies." He called the partnership with Americans "crucial" for Ukrainians.

Biden also stressed his commitment to revitalizing the two countries' strategic partnership, and supporting efforts by Zelenskiy to fight corruption and promote democratic reforms, the White House said.

President Donald Trump had emphasized a close relationship with Russian President Vladimir Putin, and resisted calls to press Russia over its interference in the United States and other countries, including Ukraine. Biden has publicly rebuked Putin since taking office, including saying last month he regarded Putin as a "killer."

Russia recalled its ambassador to the United States after that, and Russian Foreign Minister Sergey Lavrov kept up Russia's complaints about Biden's comments this week, saying Russia's relations with the United States and its allies have "hit the bottom."

Zelenskiy and Biden also discussed cooperation in strengthening democracy in Eastern Europe and containing the COVID-19 pandemic, the White House said.

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Jim Heintz contributed from Moscow.