Joe Biden has spoken with the Ukrainian president for the first time as tensions between the country and Russia continue to rise.
Mr Biden and Volodymyr Zelensky spoke on Friday as US and Nato officials warned of a Russian military build-up near eastern Ukraine.
The US president’s call with his Ukrainian counterpart, who has asked for greater western support, came more than two months after he took office.
The White House said Mr Biden “affirmed the United States’ unwavering support for Ukraine’s sovereignty and territorial integrity in the face of Russia’s ongoing aggression”.
The call took place after British foreign minister Dominic Raab said that Britain was monitoring Russian military activity near Ukraine’s border.
“I reaffirmed UK support for Ukraine’s sovereignty & territorial integrity in my call with [Ukrainian foreign minister Dmytro Kuleba] today,” Mr Raab said on Twitter.
“We are gravely concerned about Russian military activity which threatens Ukraine.”
Four Ukrainian soldiers were allegedly killed by Russian-backed separatists in eastern Ukraine earlier this week and the incident has led to fears that the long-simmering conflict could again be ignited.
“The Ukrainian side is very pleased with the call and sincerely believe President Biden prioritises restoring Ukraine’s territorial integrity and its western integration,” a source close to Mr Zelensky told Axios.
Antony Blinken, the US secretary of state, reiterated Washington’s support for Ukraine’s territorial integrity “in the face of Russia’s ongoing aggression”, in a statement on Wednesday.
The Kremlin has said that it is concerned that Ukraine will be the one who initiates wider conflict in the region.
“We express concern over the growing tension and express concern that one way or another the Ukrainian side could take provocative actions that would lead to war. We really don’t want to see that,” said Kremlin spokesman Dmitry Peskov, according to Reuters.
“I mean a civil war, which there already was there,” he added when asked to clarify by reporters.
Russia annexed Ukraine’s Crimean Peninsula in 2014 and backed a separatist insurgency in eastern Ukraine, following the ouster of the smaller country’s former Moscow-friendly president after months of protests.
It led the US and EU to impose swingeing sanctions on Moscow.
Joint chiefs chairman General Mark Milley, defence secretary Lloyd Austin and White House national security advisor Jake Sullivan have also held phone calls with their Ukrainian counterparts, according to Politico.