"There are no American soldiers at war in Ukraine. I'm determined to keep it that way, but we are standing strong with Ukraine, and we will stand with them," said the Democrat, addressing the graduating class of the prestigious West Point Military Academy.

Russian President Vladimir Putin, whom he described as "a brutal tyrant," was "certain that NATO would fracture" after he invaded his East European neighbor in February 2022, Biden said.

"Instead, the greatest defense alliance in the history of the world is stronger than ever," he said.

Last month, US lawmakers passed a long-delayed $61 billion military aid deal for Kyiv after months of squabbling in Congress as Ukrainian forces suffered setbacks on the battlefield due to ammunition and funding shortages.

Since then, Biden has ordered five tranches of military aid to be sent to Ukraine, as Russia presses its assault on the Kharkiv region.

Biden also praised the US role in the Middle East, noting that Washington is conducting "urgent diplomacy" to secure a ceasefire and bring home hostages held by the Palestinian militant group Hamas.

"Thanks to the US armed forces, we're doing what only America can do as the indispensable nation, the world's only superpower," he said.

The US president, who is campaigning for re-election in November, also urged the cadets to honour their oath to "not a political party, not to a president, but to the Constitution of the United States of America, against all enemies, foreign and domestic."

He called them "guardians of American democracy."

"Freedom is not free. It requires constant vigilance," added Biden, who is campaigning in part on the threat posed to the US political system by Trump, who has vowed to wage a crackdown on the "enemy from within" if he wins the White House again, and has avoided ruling out political violence if he loses.