Joe Biden has pledged US support for Ukraine in the face of “Russia’s ongoing aggression”, as he held his first talks with the country’s leader Volodymyr Zelensky since becoming US president.

The phone call on Friday is the most high-profile US effort to rally diplomatic support for Ukraine since Kyiv raised the alarm over a Russian military build-up along its border and in Russian-annexed Crimea.

Biden “affirmed the US’s unwavering support for Ukraine’s sovereignty and territorial integrity in the face of Russia’s ongoing aggression in the Donbas and Crimea”, the White House said in a statement after the call.

Biden also expressed support for Ukraine’s plans to tackle domestic corruption, which has regularly overshadowed the US-Ukraine relationship.

“Glad to talk to @POTUS,” Zelensky tweeted after the call, adding: “Ukraine appreciates US support on different levels.”

He continued: “My commitment to transform Ukraine, improve transparency and achieve peace is strong. The American partnership is crucial for Ukrainians.”

The call came hours after the Kremlin warned that any move to deploy US troops in Ukraine or to strengthen Nato deployments in the wider region would force Moscow to do “everything that is necessary” to protect its own security.

Four Ukrainian soldiers were killed a week ago in an attack by Russian-backed separatists in the breakaway Donbas region, marking the largest single day fatalities for government troops since the latest shaky ceasefire took effect in July. An estimated 13,000 people have been killed in seven years of fighting, which erupted after Moscow’s 2014 annexation of Crimea.

“The escalation is noticeable,” Zelensky said on Thursday, pointing to 20 soldiers killed this year. He stressed his intention to achieve peace and regain control over occupied territories through dialogue.

Russian officials have upped their rhetoric against Ukraine in recent weeks and blamed Kyiv and Washington for escalating tensions. Unverified videos shared on social media claim to show large numbers of Russian tanks and other military equipment moving close to the Ukraine border.

Sergei Lavrov, Russia’s foreign minister, this week warned that a conflict in the Donbas “will destroy Ukraine”.

Four of Biden’s top officials have assured Ukraine of US support in recent days, including Lloyd Austin, the US defence secretary who spoke with his Ukrainian counterpart Andriy Taran on Thursday.

US officials are still assessing the intentions behind the build-up of Russian troops. They are weighing whether it could be an attempt to test the Biden team’s resolve over protecting Ukraine, or reflect Kremlin efforts to unsettle Kyiv and tap patriotic sentiment among voters. Russian president Vladimir Putin’s approval rating hit a historic high after the Crimea annexation.

A senior Ukrainian official outlined three scenarios that Kyiv expects Russia could act upon in the coming weeks. One would be aggressive military exercises along Ukraine’s borders. Another involves a provocation in the breakaway regions which Moscow could blame on Ukraine. The third is a possible incursion into mainland Ukraine from Crimea to seize the canals that were shut off after Moscow annexed the peninsula.

“Putin will probe and then he will probe deeper again. He will go further and further if there’s no reaction,” said Ihor Romanenko, a former deputy head of Ukraine’s general staff.

Steven Pifer, former US ambassador to Kyiv, said that while Washington would assess if Russia was planning a military move against its neighbour, it was more likely “just the Russians trying to rattle the Ukrainians”.

“It may be kind of a test to see how the Biden administration would respond,” he added.

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